yet more sharks.

Any suggestions on enabling support for mp3 playback in Firefox ESR under Debian Buster? I ask because, you see, it would be really nice to have Duolingo usable on desktop, but I appear to have exhausted my ability to google usefully. (Everything else is trending toward working! But I have installed all the possible relevant permutations of gstreamer1.0 plugins, and...)

Web console errors on attempting to play mp3 files:

HTTP "Content-Type" of "audio/mpeg" is not supported. Load of media resource https://d1vq87e9lcf771.cloudfront.net/linguatec_de_w/5f06f63b98551aa077592a7b612d1e26 failed. 1
Cannot play media. No decoders for requested formats: audio/mpeg