cleanup: use feature detection for RTX

modules/RTC/RTCBrowserType.js

         // whether to actually enable simulcast or not).
         // TODO: the logic should be in one single place.
         return window.chrome !== undefined;
+    },
+
+    supportsRtx() {
+        return !RTCBrowserType.isFirefox();
     }
 
     // Add version getters for other browsers when needed

modules/RTC/TraceablePeerConnection.js

                         dumpSDP(answer));
                 }
 
-                if (!this.options.disableRtx && !RTCBrowserType.isFirefox()) {
+                if (!this.options.disableRtx && RTCBrowserType.supportsRtx()) {
                     answer.sdp = this.rtxModifier.modifyRtxSsrcs(answer.sdp);
                     this.trace(
                         'createAnswerOnSuccess::postTransform (rtx modifier)',
             groups: []
         };
     }
-    if (!this.options.disableRtx && !RTCBrowserType.isFirefox()) {
+    if (!this.options.disableRtx && RTCBrowserType.supportsRtx()) {
         // 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/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 && !RTCBrowserType.isFirefox()) {
+        if (!this.options.disableRtx && RTCBrowserType.supportsRtx()) {
             this.caps.addFeature('urn:ietf:rfc:4588');
         }