--- a/dom/media/tests/mochitest/sdpUtils.js
+++ b/dom/media/tests/mochitest/sdpUtils.js
@@ -1,14 +1,32 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 var sdputils = {
 
+// Finds the codec id / payload type given a codec format
+// (e.g., "VP8", "VP9/90000"). `offset` tells us which one to use in case of
+// multiple matches.
+findCodecId: function(sdp, format, offset = 0) {
+  let regex = new RegExp("rtpmap:\([0-9]+\) " + format, "gi");
+  let match;
+  for (let i = 0; i <= offset; ++i) {
+    match = regex.exec(sdp);
+    if (!match) {
+      throw new Error("Couldn't find offset " + i + " of codec " + format
+        + " while looking for offset " + offset + " in sdp:\n" + sdp);
+    }
+  }
+  // match[0] is the full matched string
+  // match[1] is the first parenthesis group
+  return match[1];
+},
+
 checkSdpAfterEndOfTrickle: function(sdp, testOptions, label) {
   info("EOC-SDP: " + JSON.stringify(sdp));
 
   ok(sdp.sdp.includes("a=end-of-candidates"), label + ": SDP contains end-of-candidates");
   sdputils.checkSdpCLineNotDefault(sdp.sdp, label);
 
   if (testOptions.rtcpmux) {
     ok(sdp.sdp.includes("a=rtcp-mux"), label + ": SDP contains rtcp-mux");
@@ -25,26 +43,35 @@ checkSdpAfterEndOfTrickle: function(sdp,
 // takes sdp in string form (or possibly a fragment, say an m-section), and
 // verifies that the default 0.0.0.0 addr is not present.
 checkSdpCLineNotDefault: function(sdpStr, label) {
   info("CLINE-NO-DEFAULT-ADDR-SDP: " + JSON.stringify(sdpStr));
   ok(!sdpStr.includes("c=IN IP4 0.0.0.0"), label + ": SDP contains non-zero IP c line");
 },
 
 // Note, we don't bother removing the fmtp lines, which makes a good test
-// for some SDP parsing issues.  It would be good to have a "removeAllButCodec(sdp, codec)" too.
+// for some SDP parsing issues. 
 removeCodec: function(sdp, codec) {
     var updated_sdp = sdp.replace(new RegExp("a=rtpmap:" + codec + ".*\\/90000\\r\\n",""),"");
     updated_sdp = updated_sdp.replace(new RegExp("(RTP\\/SAVPF.*)( " + codec + ")(.*\\r\\n)",""),"$1$3");
     updated_sdp = updated_sdp.replace(new RegExp("a=rtcp-fb:" + codec + " nack\\r\\n",""),"");
     updated_sdp = updated_sdp.replace(new RegExp("a=rtcp-fb:" + codec + " nack pli\\r\\n",""),"");
     updated_sdp = updated_sdp.replace(new RegExp("a=rtcp-fb:" + codec + " ccm fir\\r\\n",""),"");
   return updated_sdp;
 },
 
+removeAllButPayloadType: function(sdp, pt) {
+  return sdp.replace(new RegExp("m=(\\w+ \\w+) UDP/TLS/RTP/SAVPF .*" + pt + ".*\\r\\n", "gi"),
+                     "m=$1 UDP/TLS/RTP/SAVPF " + pt + "\r\n");
+},
+
+removeRtpMapForPayloadType: function(sdp, pt) {
+  return sdp.replace(new RegExp("a=rtpmap:" + pt + ".*\\r\\n", "gi"), "");
+},
+
 removeRtcpMux: function(sdp) {
   return sdp.replace(/a=rtcp-mux\r\n/g,"");
 },
 
 removeSSRCs: function(sdp) {
   return sdp.replace(/a=ssrc.*\r\n/g,"");
 },
 

--- a/dom/media/tests/mochitest/test_peerConnection_scaleResolution.html
+++ b/dom/media/tests/mochitest/test_peerConnection_scaleResolution.html
@@ -13,33 +13,31 @@
   });
 
   const pushPrefs = (...p) => SpecialPowers.pushPrefEnv({set: p});
 
   var mustRejectWith = (msg, reason, f) =>
     f().then(() => ok(false, msg),
              e => is(e.name, reason, msg));
 
-  var removeAllButCodec = (d, codec) =>
-    (d.sdp = d.sdp.replace(/m=video (\w) UDP\/TLS\/RTP\/SAVPF \w.*\r\n/,
-                           "m=video $1 UDP/TLS/RTP/SAVPF " + codec + "\r\n"), d);
-
   function testScale(codec) {
     var pc1 = new RTCPeerConnection();
     var pc2 = new RTCPeerConnection();
 
     var add = (pc, can, failed) => can && pc.addIceCandidate(can).catch(failed);
     pc1.onicecandidate = e => add(pc2, e.candidate, generateErrorCallback());
     pc2.onicecandidate = e => add(pc1, e.candidate, generateErrorCallback());
 
     info("testing scaling with " + codec);
 
     pc1.onnegotiationneeded = e =>
       pc1.createOffer()
-      .then(d => pc1.setLocalDescription(codec == "VP8" ? d : removeAllButCodec(d, 126)))
+      .then(d => pc1.setLocalDescription(codec == "VP8"
+        ? d
+        : (d.sdp = sdputils.removeAllButPayloadType(d.sdp, 126), d)))
       .then(() => pc2.setRemoteDescription(pc1.localDescription))
       .then(() => pc2.createAnswer()).then(d => pc2.setLocalDescription(d))
       .then(() => pc1.setRemoteDescription(pc2.localDescription))
       .catch(generateErrorCallback());
 
     return navigator.mediaDevices.getUserMedia({ video: true })
     .then(stream => {
       var v1 = createMediaElement('video', 'v1');