You are supposed to do feature detection, not user agent detection since it is easily spoofed, isn’t realiable, & doesn’t account for literally all the alternative UAs that can support it. This is bad/lazy practice.

Fx doesn’t always have all the features you need, but often it usually does & where I have seen this as being deployed is management saying it isn’t worth the effort to support. Just having one person on the team running Fx is usually enough to catch the game-breaking bugs.