added the experimental release note of @starting-style (#33679)

* added the experimental release note of @starting-style

* Update files/en-us/mozilla/firefox/experimental_features/index.md

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

---------

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

files/en-us/mozilla/firefox/experimental_features/index.md

@@ -1045,6 +1045,46 @@ For more details, see [Firefox bug 1823463](https://bugzil.la/1823463) for the `
   </tbody>
 </table>
 
+### `@starting-style` at-rule
+
+The CSS [`@starting-style`](/en-US/docs/Web/CSS/@starting-style) at-rule allows you to set the starting styles of an element for a CSS transition, when the element has no default initial style. This is particularly useful for elements that are hidden from view on the first paint such as [`popover`](/en-US/docs/Web/HTML/Global_attributes/popover) or ['dialog'](/en-US/docs/Web/HTML/Element/dialog). For more details, see [Firefox bug 1834876](https://bugzil.la/1834876).
+
+<table>
+  <thead>
+    <tr>
+      <th>Release channel</th>
+      <th>Version added</th>
+      <th>Enabled by default?</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <th>Nightly</th>
+      <td>127</td>
+      <td>Yes</td>
+    </tr>
+    <tr>
+      <th>Developer Edition</th>
+      <td>127</td>
+      <td>No</td>
+    </tr>
+    <tr>
+      <th>Beta</th>
+      <td>127</td>
+      <td>No</td>
+    </tr>
+    <tr>
+      <th>Release</th>
+      <td>127</td>
+      <td>No</td>
+    </tr>
+    <tr>
+      <th>Preference name</th>
+      <td colspan="2"><code>layout.css.starting-style-at-rules.enabled</code></td>
+    </tr>
+  </tbody>
+</table>
+
 ### Symmetrical `letter-spacing`
 
 The CSS `letter-spacing` property now splits the specified letter spacing evenly on both sides of each character. This is unlike the current behavior where spacing is added primarily to one side. This approach can improve text spacing, especially in mixed-directional text [Firefox bug 1891446](https://bugzil.la/1891446).
@@ -1060,7 +1100,7 @@ The CSS `letter-spacing` property now splits the specified letter spacing evenly
   <tbody>
     <tr>
       <th>Nightly</th>
-      <td>127</td>
+      <td>128</td>
       <td>Yes</td>
     </tr>
     <tr>