One datapoint: I use Slack exclusively in the browser (both Firefox and Brave), and it works well. As far as I remember on Firefox there was some limitation on video calls but that was arbitrary (user-agent check or something like that).
I hadn't tried the user agent trick, I was just frustrated enough at being on my personal laptop and having to join a call then finding Slack was being terrible.
And for no apparent good reason other than bad quality software.
I only brought it up because it is high profile web application that has an arbitrary Firefox limitation, and as a Firefox user I will name and shame bad applications.
