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.
I have the problem with Discord, that TenFourFox does not reognize the mic or the speaker. It says that version 38 or higher is required. My TenFourFox has version 45 in reality and version 60 (emulation). Both modes do not except the Mic and the speakers and refer to a newer FireFox-version, that I already have with TenFourFox FPR16. Keyboard typing is so heavy instead of hearing and speaking...Any suggestions?