It's funny because I have never seen the mentioned Firefox error on any Jira instance. The problem is mostly server-side: it's slow as hell! I'm personally using go-jira [0], but even that is slow: the problem as I said is not (only) the frontend, but the backend.
It says "missing" but it mostly talks about those existing features lacking any polish.
> and that it's super slow!
It is and it really sucks or to quote Firefox "A web page is slowing down your browser, what do you want to do, wait or stop it".