diff --git a/lib/CookieChanger.js b/lib/CookieChanger.js
index 9661550..0bc154f 100644
--- a/lib/CookieChanger.js
+++ b/lib/CookieChanger.js
@@ -4,6 +4,7 @@ const {Ci} = require("chrome");
 // export CookieChanger
 exports.CookieChanger = CookieChanger;
 exports.Cookies = Cookies;
+exports.modifyPrefCookie = modifyPrefCookie;
 
 // save all CookieChangers
 var listeners = [];
@@ -89,6 +90,34 @@ Cookies.prototype.set = function(key, value) {
 }
 
 
+/**
+ * Set single keys in the PREF cookie used by youtube to store settings
+ *
+ * @param cookies Cookies The cookies instance to modify
+ * @param key string The key to set or replace if existing, eg. "f2"
+ * @param value string The value for the given key
+ */
+function modifyPrefCookie(cookies, key, value) {
+    var entry = key + "=" + value;
+
+    if(cookies.has("PREF")) {
+        var pref = cookies.get("PREF");
+        var pattern = new RegExp(key + "=[^&]+");
+
+        if(pref.length == 0) {
+            pref = entry;
+        } else if(-1 == pref.search(pattern)) {
+            pref += "&" + entry;
+        } else {
+            pref = pref.replace(pattern, entry);
+        }
+        cookies.set("PREF", pref);
+    } else {
+        cookies.set("PREF", entry);
+    }
+}
+
+
 // Event listener that calls the other registered listeners based on
 // the active state and the given hostname.
 // The cookie header is only parsed when at least one callback is applicable.
diff --git a/lib/main.js b/lib/main.js
index 5c8807b..ee48861 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -8,7 +8,7 @@ var tabs = require("sdk/tabs");
 const {UserAgentChanger} = require("./UserAgentChanger");
 const {URIChanger} = require("./URIChanger");
 const {PluginPermissionChanger} = require("./PluginPermissionChanger");
-const {CookieChanger} = require("./CookieChanger");
+const {CookieChanger, modifyPrefCookie} = require("./CookieChanger");
 
 // Internet Explorer 10 on Windows 7
 const IEUserAgent = "Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)";
@@ -31,20 +31,7 @@ var youtubeTheaterMode = new CookieChanger("www.youtube.com", (cookies) => {
     cookies.set("wide", "1");
 });
 var youtubeHTML5Test = new CookieChanger("www.youtube.com", (cookies) => {
-    if(cookies.has("PREF")) {
-        var pref = cookies.get("PREF");
-        var pattern = /f2=\d+/;
-        if(pref.length == 0) {
-            pref = "f2=40000000";
-        } else if(-1 == pref.search(pattern)) {
-            pref += "&f2=40000000";
-        } else {
-            pref = pref.replace(pattern, "f2=40000000");
-        }
-        cookies.set("PREF", pref);
-    } else {
-        cookies.set("PREF", "f2=40000000");
-    }
+    modifyPrefCookie(cookies, "f2", "40000000");
 });
 
 function createContextMenuEntry() {
diff --git a/test/test-cookies.js b/test/test-cookies.js
index d06ba52..6b48d1b 100644
--- a/test/test-cookies.js
+++ b/test/test-cookies.js
@@ -25,4 +25,22 @@ exports.testCookieParsing = (assert) => {
     assert.equal(t("a=1; abc; b=2"), "a=1; b=2");
 };
 
+exports.testModifyPref = (assert) => {
+    var t = (testcase, key, value) => {
+        var cookies = new Cookies(testcase);
+        modifyPrefCookie(cookies, key, value);
+        return cookies.unparse();
+    };
+
+    var t1 = "PREF=f1=abc&f2=999";
+    assert.equal(t(t1, "f1", "1234"), "PREF=f1=1234&f2=999");
+    assert.equal(t(t1, "f2", "abcd"), "PREF=f1=abc&f2=abcd");
+    assert.equal(t(t1, "f3", "123"), "PREF=f1=abc&f2=999&f3=123");
+    assert.equal(t(t1, "f1", ""), "PREF=f1=&f2=999");
+    assert.equal(t(t1, "f2", ""), "PREF=f1=abc&f2=");
+
+    assert.equal(t("PREF=", "f2", "1"), "PREF=f2=1");
+    assert.equal(t("", "f2", "1"), "PREF=f2=1");
+};
+
 require('sdk/test').run(exports);