The Firefox browser’s WebRTC and MediaRecorder functionality can handle H.264 (Advanced Video Codec) coding and decoding, but only with H.264’s constrained baseline profile. Other, higher, profiles just don’t work. And in Firefox’s WebRTC implementation (v87) attempts to use higher profiles fail more-or-less silently.
Firefox works around all the patent issues with H.264 by using an automatically downloaded plugin called “OpenH264 Video Codec provided by Cisco Systems, Inc.”. It comes from the Open H264 project sponsored by Cisco.
MediaRecorder works with the MIME type video/webm; codecs="avc1.42E01F", and WebRTC uses profile-level-id='42e01f' . Other browsers allow more profile levels.