Форум поддержки Firefox

Firefox has become very slow for some two weeks om my desktop with Linux Xubuntu 18.04. Some sites can be approached without problems except for the speed, other sites are opened after some time, but freeze so that scrolling and clicking on links won't work. I tried to find an answer in the Settings......

I deinstalled firefox 98 completely from my Xubuntu 20.04.4 I then deleted all firefox related things from ~/.cache and ~/.config I rebooted I newly installed Firefox 98 from repository. Firefox 98 still does not produce sounds. Even a local file:///home/rp/Music/testfile.mp3 runs but does not produce audio output (it worked fine in Firefox 97). The Volume display is showing max. value. Firefox is even visible as stream in sound system's Volume Control / Playback (when the mp3 file is running) with 100% On my system, audio works fine with chromium browser as well as all other audio apps - as before. The earlier release Firefox 97 ran fine and produced audio output as it should. Firefox 98 currently runs without additional add-ons (no one installed besides the default Open H264 Video Codec).

What can I do to get Firefox 98 to output sound?