This is likely legacy code. Firefox used to have a lot of issues with WebRTC, so practically all video conferencing systems blocked it. Teams probably has some "block Firefox because it doesn't work properly" check that was written 5+ years ago and none of the current developers are even aware of its existence.
Well-coded ones did feature detection instead of checking the user-agent, meaning they automatically started allowing Firefox as soon as it implemented all the required features.
Feature detection is usually the way to go. If your website / webapp depends on a particular feature, check if that specific feature exists, rather than checking for particular browsers. Browser checks are still needed in some cases, for example Safari sometimes reports that it supports particular features but it really doesn't (or they're so buggy to the point where they're unusable), but that's relatively rare.