To be fair, I'm not even sure if Slack cares about web users at all. It just doesn't work that well, at least for me. (Though I obviously haven't tried it in a bit, so maybe it's good in Chrome now.)
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.