fix(AV1): scalability for FF 136+

modules/RTC/TPCUtils.js

             // only one outbound-rtp stream is present, three separate encodings have to be configured.
             || (!this.codecSettings[videoCodec].scalabilityModeEnabled && videoCodec === CodecMimeType.VP9)
 
+            // FF uses simulcast with AV1.
+            || (!this.codecSettings[videoCodec].scalabilityModeEnabled
+                && this.codecSettings[videoCodec].useSimulcast
+                && videoCodec === CodecMimeType.AV1)
+
             // When scalability is enabled, always for H.264, and only when simulcast is explicitly enabled via
             // config.js for VP9 and AV1 since full SVC is the default mode for these 2 codecs.
             || (this.codecSettings[videoCodec].scalabilityModeEnabled

modules/browser/BrowserCapabilities.js

         return this.isChromiumBased() && this.isEngineVersionLessThan(112);
     }
 
+    /**
+     * Returns true if K-SVC is supported for AV1.
+     *
+     * @returns {boolean}
+     */
+    supportsKSVCForAV1() {
+        return !this.isFirefox();
+    }
+
     /**
      * Returns true if VP9 is supported by the client on the browser. VP9 is currently disabled on Safari
      * and older versions of Firefox because of issues. Please check https://bugs.webkit.org/show_bug.cgi?id=231074 for

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.supportsKSVCForAV1(),
+        useKSVC: browser.supportsKSVCForAV1()
     },
     h264: {
         maxBitratesVideo: {