fix(Firefox): Enable RTX support on Firefox

Add RFC 4588 to the list of advertised features on Firefox.
Use RtxModifier only for cases when simulcast is achieved through SDP munging
This will ensure that RTX is negotiated whenever it is offered by the browser.

modules/RTC/TraceablePeerConnection.js

                     dumpSDP(resultSdp));
             }
 
-            if (!this.options.disableRtx && browser.supportsRtx()) {
+            if (!this.options.disableRtx && browser.usesSdpMungingForSimulcast()) {
                 // eslint-disable-next-line no-param-reassign
                 resultSdp = new RTCSessionDescription({
                     type: resultSdp.type,
             groups: []
         };
     }
-    if (!this.options.disableRtx && browser.supportsRtx()) {
+    if (!this.options.disableRtx) {
         // Specifically use a for loop here because we'll
         //  be adding to the list we're iterating over, so we
         //  only want to iterate through the items originally

modules/browser/BrowserCapabilities.js

         return this.isFirefox() || this.isSafari();
     }
 
-    /**
-     * Checks whether the browser supports RTX.
-     *
-     * @returns {boolean}
-     */
-    supportsRtx() {
-        return !this.isFirefox();
-    }
-
     /**
      * Returns whether or not the current browser can support capturing video,
      * be it camera or desktop, and displaying received video.
      * @returns {boolean}
      */
     usesSdpMungingForSimulcast() {
-        return this.isChromiumBased() || this.isSafari();
+        return this.isChromiumBased() || this.isReactNative() || this.isSafari();
     }
 
     /**

modules/xmpp/xmpp.js

         this.caps.addFeature('urn:xmpp:jingle:apps:rtp:audio');
         this.caps.addFeature('urn:xmpp:jingle:apps:rtp:video');
 
-        if (!this.options.disableRtx && browser.supportsRtx()) {
+        if (!this.options.disableRtx) {
             this.caps.addFeature('urn:ietf:rfc:4588');
         }