I created a test account at discordapp.com. Logging into https://discordapp.com/activity I also get the black screen, but I'm not getting the "this is undefined" JS error which might lead to issue 533 you're seeing. So it may be something else.
I can log in with Safari/LWK (r226588) just fine, no crash.
I'm pretty sure – as you've been able to log in until recently – that they introduced something at the front end which we don't support (yet).
There is a construct in the code "for(const n in this.payload)" which is probably https://github.com/classilla/tenfourfox/issues/541 . The solution there was very hacky and I chose not to implement it at the time since it didn't clearly fix anything, but I can reconsider it for FPR15 (too late for 14). However, no guarantees, since there may be other broken things that this just unmasks.
an update (sort of) - I was importing some bookmarks into Safari WK on a whim I tried the imported Discord link and it went thru. Then I clicked on the field to reset my password and in TFF accessed the email and clicked the reset button, and it opened Discord and informed me I reset my password - again just a black screen. :-(
BUT it did work and I was able to login with Safari WK.
so some progress, I'm guessing that b4 I copy/paste the link and Safari didn't like that and crashed - still not sure.