To summarize my current understanding: Slack's video conferencing service only support Chrome's non-standard Plan B format for WebRTC calls. And our attempts to talk to Slack about changing that were kindly rejected (we met with them shortly after they had launched the calling feature). And my repeated questions in public events about adding Firefox support were also turned down.
Source: https://bugzilla.mozilla.org/show_bug.cgi?id=1626121#c4
Further clarification from last year https://bugzilla.mozilla.org/show_bug.cgi?id=1626121#c10
I have seen people mention that they have tried few more times to contact them, but Slack weren't interested.
My understanding as to why it works now, correct me if I am wrong, is because of Slack moving to using AWS Chime SDK for calls which has Firefox support and just faking the UA to Chrome tricks Slack into loads the necessary SDK/JS for calls and Chime SDK takes care of the rest.