Again, Atlassian products don't work in Firefox.
Dunno who's to blame - Atlassian or Mozilla. All I know is this sucks! Can't stand Edge and the sorts.

try disabling resistFingerprinting flag in about:config

@kiki

I'll try that next time I run into the same problem.

This time around I managed to get past by spamclicking the login window until it finally found its marbles.

The most common web browsers are so interchangeable nowadays that when something doesn't work in one of them, it would seem almost sabotage.

I remember JIRA working quite well in Firefox...

...though I haven't dealt w/ it in at least 5 years && it's just one of several Atlassian's software, so...

also, I assume that you clicked the shield icon to the left of the address bar and disabled protection there. also, check if you have webgl and webrtc disabled in about:config

@kiki

Actually I didn't. It somehow works now, so I don't bother.

Every time Atlassian makes an update, one needs to open the dev console, select the Network tab and click Update - despite having disabled cache. I consider this a flaw in Firefox.