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It refuses calls as "not supported in this browser".

Are you user-agent spoofing?

I'm also using Slack on Firefox and I'm not doing any user agent spoofing.

Calls?

That's what I had issues with, and what has been confirmed by multiple other people in this thread unless you spoof your user agent.

I've never used calls. Never even considered using it. So could be that feature isn't working.

For the rest, slack works well enough on FF. Often faster than the desktop app because the latter is so resource hungry, it becomes slow.