diff --git a/dotnet/AGENTS.md b/dotnet/AGENTS.md
index 81c7b1766a874..2c48e93d2f7db 100644
--- a/dotnet/AGENTS.md
+++ b/dotnet/AGENTS.md
@@ -28,6 +28,14 @@ _logger.Debug("diagnostic: request details for debugging");
 [Obsolete("Use NewMethod instead")]
 public void OldMethod() { }
 ```
+When code inside the assembly must still reference an obsolete member (e.g. a field
+or method the obsolete API is built on), wrap just that usage to keep the build
+warning-clean (see `UserPromptHandler.cs`):
+```csharp
+#pragma warning disable CS0618 // Type or member is obsolete
+this.legacyThing.DoWork();
+#pragma warning restore CS0618 // Type or member is obsolete
+```
 
 ### Async patterns
 The codebase is migrating to async
diff --git a/dotnet/src/webdriver/Firefox/FirefoxExtension.cs b/dotnet/src/webdriver/Firefox/FirefoxExtension.cs
index be47e57e8a8f5..ce75367008141 100644
--- a/dotnet/src/webdriver/Firefox/FirefoxExtension.cs
+++ b/dotnet/src/webdriver/Firefox/FirefoxExtension.cs
@@ -29,6 +29,7 @@ namespace OpenQA.Selenium.Firefox;
 /// <summary>
 /// Provides the ability to install extensions into a <see cref="FirefoxProfile"/>.
 /// </summary>
+[Obsolete("Use FirefoxDriver.InstallAddOnFromFile instead.")]
 public class FirefoxExtension
 {
     private const string EmNamespaceUri = "http://www.mozilla.org/2004/em-rdf#";
diff --git a/dotnet/src/webdriver/Firefox/FirefoxProfile.cs b/dotnet/src/webdriver/Firefox/FirefoxProfile.cs
index 17619687051e2..ce0547d767450 100644
--- a/dotnet/src/webdriver/Firefox/FirefoxProfile.cs
+++ b/dotnet/src/webdriver/Firefox/FirefoxProfile.cs
@@ -33,7 +33,9 @@ public class FirefoxProfile
     private readonly string? sourceProfileDir;
     private readonly bool deleteSource;
     private readonly Preferences profilePreferences;
+#pragma warning disable CS0618 // Type or member is obsolete
     private readonly Dictionary<string, FirefoxExtension> extensions = new Dictionary<string, FirefoxExtension>();
+#pragma warning restore CS0618 // Type or member is obsolete
 
     /// <summary>
     /// Initializes a new instance of the <see cref="FirefoxProfile"/> class.
@@ -103,11 +105,14 @@ public static FirefoxProfile FromBase64String(string base64)
     /// </summary>
     /// <param name="extensionToInstall">The path to the new extension</param>
     /// <exception cref="ArgumentNullException">If <paramref name="extensionToInstall"/> is <see langword="null"/>.</exception>
+    [Obsolete("Use FirefoxDriver.InstallAddOnFromFile instead.")]
     public void AddExtension(string extensionToInstall)
     {
         ArgumentNullException.ThrowIfNull(extensionToInstall);
 
+#pragma warning disable CS0618 // Type or member is obsolete
         this.extensions.Add(Path.GetFileNameWithoutExtension(extensionToInstall), new FirefoxExtension(extensionToInstall));
+#pragma warning restore CS0618 // Type or member is obsolete
     }
 
     /// <summary>
@@ -236,10 +241,12 @@ private void DeleteLockFiles(string profileDirectory)
     /// </summary>
     private void InstallExtensions(string profileDirectory)
     {
+#pragma warning disable CS0618 // Type or member is obsolete
         foreach (string extensionKey in this.extensions.Keys)
         {
             this.extensions[extensionKey].Install(profileDirectory);
         }
+#pragma warning restore CS0618 // Type or member is obsolete
     }
 
     /// <summary>
diff --git a/java/src/org/openqa/selenium/firefox/ClasspathExtension.java b/java/src/org/openqa/selenium/firefox/ClasspathExtension.java
index ce2dc1283dd5a..0a5d68e4cc4af 100644
--- a/java/src/org/openqa/selenium/firefox/ClasspathExtension.java
+++ b/java/src/org/openqa/selenium/firefox/ClasspathExtension.java
@@ -25,6 +25,10 @@
 import org.openqa.selenium.WebDriverException;
 import org.openqa.selenium.io.FileHandler;
 
+/**
+ * @deprecated Use {@link HasExtensions#installExtension} instead.
+ */
+@Deprecated(forRemoval = true)
 public class ClasspathExtension implements Extension {
   private final Class<?> loadResourcesUsing;
   private final String loadFrom;
diff --git a/java/src/org/openqa/selenium/firefox/Extension.java b/java/src/org/openqa/selenium/firefox/Extension.java
index 629adb4762724..86a8d417e9a5b 100644
--- a/java/src/org/openqa/selenium/firefox/Extension.java
+++ b/java/src/org/openqa/selenium/firefox/Extension.java
@@ -20,6 +20,10 @@
 import java.io.File;
 import java.io.IOException;
 
+/**
+ * @deprecated Use {@link HasExtensions#installExtension} instead.
+ */
+@Deprecated(forRemoval = true)
 public interface Extension {
   void writeTo(File parentDirectory) throws IOException;
 }
diff --git a/java/src/org/openqa/selenium/firefox/FileExtension.java b/java/src/org/openqa/selenium/firefox/FileExtension.java
index db9e755f00527..57746c152750a 100644
--- a/java/src/org/openqa/selenium/firefox/FileExtension.java
+++ b/java/src/org/openqa/selenium/firefox/FileExtension.java
@@ -47,6 +47,10 @@
 import org.w3c.dom.Document;
 import org.w3c.dom.Node;
 
+/**
+ * @deprecated Use {@link HasExtensions#installExtension} instead.
+ */
+@Deprecated(forRemoval = true)
 public class FileExtension implements Extension {
   private static final String EM_NAMESPACE_URI = "http://www.mozilla.org/2004/em-rdf#";
 
diff --git a/java/src/org/openqa/selenium/firefox/FirefoxProfile.java b/java/src/org/openqa/selenium/firefox/FirefoxProfile.java
index 023bf8571cf40..7fbfe921b6c21 100644
--- a/java/src/org/openqa/selenium/firefox/FirefoxProfile.java
+++ b/java/src/org/openqa/selenium/firefox/FirefoxProfile.java
@@ -38,7 +38,10 @@ public class FirefoxProfile {
   private static final String ACCEPT_UNTRUSTED_CERTS_PREF = "webdriver_accept_untrusted_certs";
   private static final String ASSUME_UNTRUSTED_ISSUER_PREF = "webdriver_assume_untrusted_issuer";
   private final Preferences additionalPrefs;
+
+  @SuppressWarnings("deprecation")
   private final Map<String, Extension> extensions = new HashMap<>();
+
   private @Nullable final File model;
   private boolean loadNoFocusLib;
   private boolean acceptUntrustedCerts;
@@ -145,6 +148,10 @@ public boolean containsWebDriverExtension() {
     return extensions.containsKey("webdriver");
   }
 
+  /**
+   * @deprecated Use {@link HasExtensions#installExtension} instead.
+   */
+  @Deprecated(forRemoval = true)
   public void addExtension(Class<?> loadResourcesUsing, String loadFrom) {
     // Is loadFrom a file?
     File file = new File(loadFrom);
@@ -160,11 +167,17 @@ public void addExtension(Class<?> loadResourcesUsing, String loadFrom) {
    * Attempt to add an extension to install into this instance.
    *
    * @param extensionToInstall File pointing to the extension
+   * @deprecated Use {@link HasExtensions#installExtension} instead.
    */
+  @Deprecated(forRemoval = true)
   public void addExtension(File extensionToInstall) {
     addExtension(extensionToInstall.getName(), new FileExtension(extensionToInstall));
   }
 
+  /**
+   * @deprecated Use {@link HasExtensions#installExtension} instead.
+   */
+  @Deprecated(forRemoval = true)
   public void addExtension(String key, Extension extension) {
     String name = deriveExtensionName(key);
     extensions.put(name, extension);
@@ -248,7 +261,9 @@ public void deleteExtensionsCacheIfItExists(File profileDir) {
    * even if native events are disabled.
    *
    * @return Whether the no focus library should always be loaded for Firefox on Linux.
+   * @deprecated Native events are no longer supported.
    */
+  @Deprecated(forRemoval = true)
   public boolean shouldLoadNoFocusLib() {
     return loadNoFocusLib;
   }
@@ -257,7 +272,9 @@ public boolean shouldLoadNoFocusLib() {
    * Sets whether the no focus library should always be loaded on Linux.
    *
    * @param loadNoFocusLib Whether to always load the no focus library.
+   * @deprecated Native events are no longer supported.
    */
+  @Deprecated(forRemoval = true)
   public FirefoxProfile setAlwaysLoadNoFocusLib(boolean loadNoFocusLib) {
     this.loadNoFocusLib = loadNoFocusLib;
     return this;
@@ -268,7 +285,9 @@ public FirefoxProfile setAlwaysLoadNoFocusLib(boolean loadNoFocusLib) {
    * authority or are generally untrusted. This is set to true by default.
    *
    * @param acceptUntrustedSsl Whether untrusted SSL certificates should be accepted.
+   * @deprecated Use {@link FirefoxOptions#setAcceptInsecureCerts(boolean)} instead.
    */
+  @Deprecated(forRemoval = true)
   public FirefoxProfile setAcceptUntrustedCertificates(boolean acceptUntrustedSsl) {
     this.acceptUntrustedCerts = acceptUntrustedSsl;
     return this;
@@ -287,7 +306,9 @@ public FirefoxProfile setAcceptUntrustedCertificates(boolean acceptUntrustedSsl)
    * production certificate served in a testing environment) set this to false.
    *
    * @param untrustedIssuer whether to assume untrusted issuer or not.
+   * @deprecated Use {@link FirefoxOptions#setAcceptInsecureCerts(boolean)} instead.
    */
+  @Deprecated(forRemoval = true)
   public FirefoxProfile setAssumeUntrustedCertificateIssuer(boolean untrustedIssuer) {
     this.untrustedCertIssuer = untrustedIssuer;
     return this;
@@ -347,6 +368,7 @@ protected void copyModel(@Nullable File sourceDir, File profileDir) throws IOExc
     FileHandler.copy(sourceDir, profileDir);
   }
 
+  @SuppressWarnings("deprecation")
   protected void installExtensions(File parentDir) throws IOException {
     File extensionsDir = new File(parentDir, "extensions");
 
diff --git a/javascript/selenium-webdriver/firefox.js b/javascript/selenium-webdriver/firefox.js
index d97500398fb99..e18d6712f9880 100644
--- a/javascript/selenium-webdriver/firefox.js
+++ b/javascript/selenium-webdriver/firefox.js
@@ -317,6 +317,7 @@ class Options extends Capabilities {
    *
    * @param {...string} paths The paths to the extension XPI files to install.
    * @return {!Options} A self reference.
+   * @deprecated Use {@link Driver#installAddon} instead.
    */
   addExtensions(...paths) {
     this.profile_().addExtensions(paths)
diff --git a/rb/lib/selenium/webdriver/firefox/profile.rb b/rb/lib/selenium/webdriver/firefox/profile.rb
index 55d7d7bc1afb6..5166d211a7b0d 100644
--- a/rb/lib/selenium/webdriver/firefox/profile.rb
+++ b/rb/lib/selenium/webdriver/firefox/profile.rb
@@ -40,7 +40,6 @@ class Profile
         LOCK_FILES = %w[.parentlock parent.lock lock].freeze
 
         attr_reader :name, :log_file
-        attr_writer :secure_ssl, :load_no_focus_lib
 
         class << self
           def ini
@@ -110,19 +109,28 @@ def []=(key, value)
         end
 
         def port=(port)
+          WebDriver.logger.deprecate('Firefox::Profile#port=', 'the Service class', id: :firefox_profile)
           self[WEBDRIVER_PREFS[:port]] = port
         end
 
+        def secure_ssl=(value)
+          WebDriver.logger.deprecate('Firefox::Profile#secure_ssl=', id: :firefox_profile)
+          @secure_ssl = value
+        end
+
+        def load_no_focus_lib=(value)
+          WebDriver.logger.deprecate('Firefox::Profile#load_no_focus_lib=', id: :firefox_profile)
+          @load_no_focus_lib = value
+        end
+
         def log_file=(file)
           @log_file = file
           self[WEBDRIVER_PREFS[:log_file]] = file
         end
 
-        #
-        # Add the extension (directory, .zip or .xpi) at the given path to the profile.
-        #
-
         def add_extension(path, name = extension_name_for(path))
+          WebDriver.logger.deprecate('Firefox::Profile#add_extension', 'Driver#install_addon',
+                                     id: :firefox_profile)
           @extensions[name] = Extension.new(path)
         end
 
diff --git a/rb/spec/integration/selenium/webdriver/firefox/profile_spec.rb b/rb/spec/integration/selenium/webdriver/firefox/profile_spec.rb
index ae54cd5167234..abf6586a8a20d 100644
--- a/rb/spec/integration/selenium/webdriver/firefox/profile_spec.rb
+++ b/rb/spec/integration/selenium/webdriver/firefox/profile_spec.rb
@@ -45,6 +45,12 @@ module Firefox
             end
           end
         end
+
+        it 'ships preferences from an existing profile directory' do
+          reset_driver!(profile: described_class.new(profile.layout_on_disk)) do |driver|
+            expect { wait(5).until { driver.find_element(id: 'oneline') } }.not_to raise_error
+          end
+        end
       end
     end # Firefox
   end # WebDriver
diff --git a/rb/spec/unit/selenium/webdriver/firefox/profile_spec.rb b/rb/spec/unit/selenium/webdriver/firefox/profile_spec.rb
index 7a460764f227e..154133fc01861 100644
--- a/rb/spec/unit/selenium/webdriver/firefox/profile_spec.rb
+++ b/rb/spec/unit/selenium/webdriver/firefox/profile_spec.rb
@@ -122,28 +122,6 @@ def read_generated_prefs(from = nil)
 
           expect(read_generated_prefs).to include('user_pref("network.proxy.type", 4)')
         end
-
-        it 'can install extension' do
-          firebug = File.expand_path('../../../../../../third_party/firebug/firebug-1.5.0-fx.xpi', __dir__)
-          profile.add_extension(firebug)
-          extension_directory = File.expand_path('extensions/firebug@software.joehewitt.com', profile.layout_on_disk)
-          expect(Dir.exist?(extension_directory)).to be(true)
-        end
-
-        it 'can install web extension without id' do
-          mooltipass = File.expand_path('../../../../../../third_party/firebug/mooltipass-1.1.87.xpi', __dir__)
-          profile.add_extension(mooltipass)
-          extension_directory = File.expand_path('extensions/MooltipassExtension@1.1.87', profile.layout_on_disk)
-          expect(Dir.exist?(extension_directory)).to be(true)
-        end
-
-        it 'can install web extension with id' do
-          ext = File.expand_path('../../../../../../third_party/firebug/favourite_colour-1.1-an+fx.xpi', __dir__)
-          profile.add_extension(ext)
-          extension_directory = File.expand_path('extensions/favourite-colour-examples@mozilla.org',
-                                                 profile.layout_on_disk)
-          expect(Dir.exist?(extension_directory)).to be(true)
-        end
       end
     end # Firefox
   end # WebDriver