fix(VP9): Disable for FF.

Disables VP9 for FF since the simulcast  is disabled with FF flag by default. Introduces a few fixes so that if the FF flag is enabled simulcast works. This should also help us to be ready to enable VP9 once the  FF config flag for simulcast for VP9 is enabled by default.

modules/RTC/TPCUtils.js

 
         return videoCodec === CodecMimeType.VP8 // VP8 always
 
-            // K-SVC mode for VP9 when no scalability mode is set. Though only one outbound-rtp stream is present,
-            // three separate encodings have to be configured.
+            // For FF: scalabilityModeEnabled is not supported and we have to use simulcast.
+            // For other browsers we use K-SVC mode for VP9 when no scalability mode is set. Although
+            // only one outbound-rtp stream is present, three separate encodings have to be configured.
             || (!this.codecSettings[videoCodec].scalabilityModeEnabled && videoCodec === CodecMimeType.VP9)
 
             // When scalability is enabled, always for H.264, and only when simulcast is explicitly enabled via

modules/RTC/TraceablePeerConnection.js

     // case they were set when the endpoint was encoding video using the other codecs before switching over to VP9
     // K-SVC codec.
     if (codec === CodecMimeType.VP9
+        && browser.supportsKSVCForVP9()
         && this.isSpatialScalabilityOn()
         && !this.tpcUtils.codecSettings[codec].scalabilityModeEnabled) {
         scaleFactors = scaleFactors.map(() => undefined);

modules/browser/BrowserCapabilities.js

      * details.
      */
     supportsVP9() {
-        return !(this.isWebKitBased() || (this.isFirefox() && this.isVersionLessThan('136')));
+        // Keep this disabled for FF because simulcast is disabled by default.
+        // For versions 136+ if the media.webrtc.simulcast.vp9.enabled config is set to true it will work.
+        // TODO: enable for FF with version 136+ once media.webrtc.simulcast.vp9.enabled is set to true by default.
+        return !(this.isWebKitBased() || this.isFirefox());
+    }
+
+    /**
+     * Returns true if K-SVC is supported for VP9.
+     *
+     * @returns {boolean}
+     */
+    supportsKSVCForVP9() {
+        return !this.isFirefox();
     }
 
     /**

service/RTC/StandardVideoQualitySettings.ts

             none: 0
         },
         scalabilityModeEnabled: browser.supportsScalabilityModeAPI(),
-        useSimulcast: false, // defaults to SVC.
-        useKSVC: true // defaults to L3T3_KEY for SVC mode.
+        useSimulcast: !browser.supportsKSVCForVP9(),
+        useKSVC: browser.supportsKSVCForVP9()
     }
 };