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diff --git a/dom/media/webrtc/tests/mochitests/simulcast.js b/dom/media/webrtc/tests/mochitests/simulcast.js
new file mode 100644
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+++ b/dom/media/webrtc/tests/mochitests/simulcast.js
@@ -0,0 +1,232 @@
+"use strict";
+/* Helper functions to munge SDP and split the sending track into
+ * separate tracks on the receiving end. This can be done in a number
+ * of ways, the one used here uses the fact that the MID and RID header
+ * extensions which are used for packet routing share the same wire
+ * format. The receiver interprets the rids from the sender as mids
+ * which allows receiving the different spatial resolutions on separate
+ * m-lines and tracks.
+ */
+
+// Borrowed from wpt, with some dependencies removed.
+
+const ridExtensions = [
+  "urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id",
+  "urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id",
+];
+
+function ridToMid(description, rids) {
+  const sections = SDPUtils.splitSections(description.sdp);
+  const dtls = SDPUtils.getDtlsParameters(sections[1], sections[0]);
+  const ice = SDPUtils.getIceParameters(sections[1], sections[0]);
+  const rtpParameters = SDPUtils.parseRtpParameters(sections[1]);
+  const setupValue = description.sdp.match(/a=setup:(.*)/)[1];
+  const directionValue =
+    description.sdp.match(/a=sendrecv|a=sendonly|a=recvonly|a=inactive/) ||
+    "a=sendrecv";
+  const mline = SDPUtils.parseMLine(sections[1]);
+
+  // Skip mid extension; we are replacing it with the rid extmap
+  rtpParameters.headerExtensions = rtpParameters.headerExtensions.filter(
+    ext => ext.uri != "urn:ietf:params:rtp-hdrext:sdes:mid"
+  );
+
+  for (const ext of rtpParameters.headerExtensions) {
+    if (ext.uri == "urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id") {
+      ext.uri = "urn:ietf:params:rtp-hdrext:sdes:mid";
+    }
+  }
+
+  // Filter rtx as we have no way to (re)interpret rrid.
+  // Not doing this makes probing use RTX, it's not understood and ramp-up is slower.
+  rtpParameters.codecs = rtpParameters.codecs.filter(
+    c => c.name.toUpperCase() !== "RTX"
+  );
+
+  if (!rids) {
+    rids = Array.from(description.sdp.matchAll(/a=rid:(.*) send/g)).map(
+      r => r[1]
+    );
+  }
+
+  let sdp =
+    SDPUtils.writeSessionBoilerplate() +
+    SDPUtils.writeDtlsParameters(dtls, setupValue) +
+    SDPUtils.writeIceParameters(ice) +
+    "a=group:BUNDLE " +
+    rids.join(" ") +
+    "\r\n";
+  const baseRtpDescription = SDPUtils.writeRtpDescription(
+    mline.kind,
+    rtpParameters
+  );
+  for (const rid of rids) {
+    sdp +=
+      baseRtpDescription +
+      "a=mid:" +
+      rid +
+      "\r\n" +
+      "a=msid:rid-" +
+      rid +
+      " rid-" +
+      rid +
+      "\r\n";
+    sdp += directionValue + "\r\n";
+  }
+  return sdp;
+}
+
+function midToRid(description, localDescription, rids) {
+  const sections = SDPUtils.splitSections(description.sdp);
+  const dtls = SDPUtils.getDtlsParameters(sections[1], sections[0]);
+  const ice = SDPUtils.getIceParameters(sections[1], sections[0]);
+  const rtpParameters = SDPUtils.parseRtpParameters(sections[1]);
+  const setupValue = description.sdp.match(/a=setup:(.*)/)[1];
+  const directionValue =
+    description.sdp.match(/a=sendrecv|a=sendonly|a=recvonly|a=inactive/) ||
+    "a=sendrecv";
+  const mline = SDPUtils.parseMLine(sections[1]);
+
+  // Skip rid extensions; we are replacing them with the mid extmap
+  rtpParameters.headerExtensions = rtpParameters.headerExtensions.filter(
+    ext => !ridExtensions.includes(ext.uri)
+  );
+
+  for (const ext of rtpParameters.headerExtensions) {
+    if (ext.uri == "urn:ietf:params:rtp-hdrext:sdes:mid") {
+      ext.uri = "urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id";
+    }
+  }
+
+  const localMid = localDescription
+    ? SDPUtils.getMid(SDPUtils.splitSections(localDescription.sdp)[1])
+    : "0";
+
+  if (!rids) {
+    rids = [];
+    for (let i = 1; i < sections.length; i++) {
+      rids.push(SDPUtils.getMid(sections[i]));
+    }
+  }
+
+  let sdp =
+    SDPUtils.writeSessionBoilerplate() +
+    SDPUtils.writeDtlsParameters(dtls, setupValue) +
+    SDPUtils.writeIceParameters(ice) +
+    "a=group:BUNDLE " +
+    localMid +
+    "\r\n";
+  sdp += SDPUtils.writeRtpDescription(mline.kind, rtpParameters);
+  // Although we are converting mids to rids, we still need a mid.
+  // The first one will be consistent with trickle ICE candidates.
+  sdp += "a=mid:" + localMid + "\r\n";
+  sdp += directionValue + "\r\n";
+
+  for (const rid of rids) {
+    const stringrid = String(rid); // allow integers
+    const choices = stringrid.split(",");
+    choices.forEach(choice => {
+      sdp += "a=rid:" + choice + " recv\r\n";
+    });
+  }
+  if (rids.length) {
+    sdp += "a=simulcast:recv " + rids.join(";") + "\r\n";
+  }
+
+  return sdp;
+}
+
+async function doOfferToSendSimulcast(offerer, answerer) {
+  await offerer.setLocalDescription();
+
+  // Is this a renegotiation? If so, we cannot remove (or reorder!) any mids,
+  // even if some rids have been removed or reordered.
+  let mids = [];
+  if (answerer.localDescription) {
+    // Renegotiation. Mids must be the same as before, because renegotiation
+    // can never remove or reorder mids, nor can it expand the simulcast
+    // envelope.
+    mids = [...answerer.localDescription.sdp.matchAll(/a=mid:(.*)/g)].map(
+      e => e[1]
+    );
+  } else {
+    // First negotiation; the mids will be exactly the same as the rids
+    const simulcastAttr = offerer.localDescription.sdp.match(
+      /a=simulcast:send (.*)/
+    );
+    if (simulcastAttr) {
+      mids = simulcastAttr[1].split(";");
+    }
+  }
+
+  const nonSimulcastOffer = ridToMid(offerer.localDescription, mids);
+  await answerer.setRemoteDescription({
+    type: "offer",
+    sdp: nonSimulcastOffer,
+  });
+}
+
+async function doAnswerToRecvSimulcast(offerer, answerer, rids) {
+  await answerer.setLocalDescription();
+  const simulcastAnswer = midToRid(
+    answerer.localDescription,
+    offerer.localDescription,
+    rids
+  );
+  await offerer.setRemoteDescription({ type: "answer", sdp: simulcastAnswer });
+}
+
+async function doOfferToRecvSimulcast(offerer, answerer, rids) {
+  await offerer.setLocalDescription();
+  const simulcastOffer = midToRid(
+    offerer.localDescription,
+    answerer.localDescription,
+    rids
+  );
+  await answerer.setRemoteDescription({ type: "offer", sdp: simulcastOffer });
+}
+
+async function doAnswerToSendSimulcast(offerer, answerer) {
+  await answerer.setLocalDescription();
+
+  // See which mids the offerer had; it will barf if we remove or reorder them
+  const mids = [...offerer.localDescription.sdp.matchAll(/a=mid:(.*)/g)].map(
+    e => e[1]
+  );
+
+  const nonSimulcastAnswer = ridToMid(answerer.localDescription, mids);
+  await offerer.setRemoteDescription({
+    type: "answer",
+    sdp: nonSimulcastAnswer,
+  });
+}
+
+async function doOfferToSendSimulcastAndAnswer(offerer, answerer, rids) {
+  await doOfferToSendSimulcast(offerer, answerer);
+  await doAnswerToRecvSimulcast(offerer, answerer, rids);
+}
+
+async function doOfferToRecvSimulcastAndAnswer(offerer, answerer, rids) {
+  await doOfferToRecvSimulcast(offerer, answerer, rids);
+  await doAnswerToSendSimulcast(offerer, answerer);
+}
+
+// This would be useful for cases other than simulcast, but we do not use it
+// anywhere else right now, nor do we have a place for wpt-friendly helpers at
+// the moment.
+function createPlaybackElement(track) {
+  const elem = document.createElement(track.kind);
+  elem.autoplay = true;
+  elem.srcObject = new MediaStream([track]);
+  elem.id = track.id;
+  return elem;
+}
+
+async function getPlaybackWithLoadedMetadata(track) {
+  const elem = createPlaybackElement(track);
+  return new Promise(resolve => {
+    elem.addEventListener("loadedmetadata", () => {
+      resolve(elem);
+    });
+  });
+}