I have the same issue, can't join a single fireteam, turned all my extensions off, thinking it might be the problem, but no, it wasn't.
I've just been having the same issue actually and was coming to the forums to see if anyone else was dealing with this too. It's the same for myself, where I can see the fireteams on the list and even go into them and see OTHER people joining them, but the moment I try to click on which class I want to use in the fireteam and join, nothing happens. It's like the join button itself is just broken, so you sit there unable to join at all. Would like a fix as well lol