@freemo Then it seems it was (now it works again) on my side somehow.

When I connected with firefox from my pinephone it showed the loading screen for a short while and then it was replaced by a message which said more or less "Failed to work with your browser, please use a dedicated client.". I tried a couple of times, including several browser restarts and visiting pages in between, but it kept happening, so I asked about updates. Now that I tried again a couple hours later it just works, no idea why...

Sorry for the unnecessary commotion.