I found that sometime in 2021 iPhone videos exported to .mov files had no audio in Firefox, the speaker icon on the video player is crossed out indicating the volume is set to zero but clicking on it has no effect. The Brave browser for example was fine. It turns out that the default audio is raw PCM, this seems to have changed as older videos do not have this problem. The audio can be transcoded to MP3 which works fine using ffmepg. The command below copies the video unaltered to the new container and only trancodes the audio so is very fast
$ ffmpeg -i input.mov -c:v copy -c:a mp3 output.mkvVersion is
$ ffmpeg -version
ffmpeg version 4.4.2-0ubuntu0.22.04.1 Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 11 (Ubuntu 11.2.0-19ubuntu1)