Merge pull request #357 from bbaldino/disable_rtx_for_ffox

don't add rtx streams on firefox

modules/xmpp/TraceablePeerConnection.js

                         dumpSDP(answer));
                 }
 
-                if (!this.session.room.options.disableRtx) {
+                if (!this.session.room.options.disableRtx && !RTCBrowserType.isFirefox()) {
                     answer.sdp = this.rtxModifier.modifyRtxSsrcs(answer.sdp);
                     this.trace(
                         'createAnswerOnSuccess::postTransform (rtx modifier)',