hi, thanks for reporting this problem. are you able to reproduce the problem with a new profile in nightly as well? Profile Manager - Create, remove or switch Firefox profiles

if yes, could you try to use the mozregression tool from https://mozilla.github.io/mozregression/ in order to narrow down which change introduced this problem (there is a howto video on the website explaining the first steps there)?

Cannot run Nightly with a new profile because the profile manager for it will not start either (I can start the profile manager of the regular FF build though). Also could not start Nightly in safe mode

I ran mozregression, and got two sets of build windows for failure: one for nightly, one when it reran itself for "autoland" builds.

9:09.57 INFO: Narrowed inbound regression window from [48cc597d, f8b2114a] (3 builds) to [62308095, f8b2114a] (2 builds) (~1 steps left)
9:09.57 INFO: No more inbound revisions, bisection finished.
9:09.58 INFO: Last good revision: 62308095ad0df8227a2dab58f08f3d7f880cfe43
9:09.58 INFO: First bad revision: f8b2114ab5123475cebec470ead6136214c1d10c
9:09.58 INFO: Pushlog:

https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=62308095ad0df8227a2dab58f08f3d7f880cfe43&tochange=f8b2114ab5123475cebec470ead6136214c1d10c

and

14:53.93 INFO: Narrowed inbound regression window from [d86f6268, 6e4da634] (4 builds) to [23841605, 6e4da634] (2 builds) (~1 steps left) 14:53.94 INFO: No more inbound revisions, bisection finished. 14:53.94 INFO: Last good revision: 23841605d322bd069d28d5dfdd73e96247d1f130 14:53.95 INFO: First bad revision: 6e4da634fab4c1e70046fb6a6111cc2575667d62 14:53.99 INFO: Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=23841605d322bd069d28d5dfdd73e96247d1f130&tochange=6e4da634fab4c1e70046fb6a6111cc2575667d62

thanks a bunch for digging that up. is the firefox.exe process just sitting in your windows task manager but just no user interface showing up or is the process getting killed again shortly after you're trying to launch the browser?

edit: and is firefox nightly able to start when you temporarily disable your security software?

Killed immediately. I get a flash of the blue windows spinning loading circle as a mouse cursor. Wow I fell for the oldest trick in the book. It's a work laptop so I'm technically not able to disable one of the security applications, but I just nuked it out of the task manager and Nightly launched correctly before it had a chance to restart. So dumb. I'll talk to our IT about getting it whitelisted somehow, or at least seeing the logs why it's killed, but any idea why only the newest Nightly version would run into this issue? The security application in question is CbDefender

your mozregression log narrowed it down to this recent change: https://bugzilla.mozilla.org/show_bug.cgi?id=1488554 apparently this doesn't play well with something on your system (most likely security software). next to CbDefender your system details also showed the presence of ESET Endpoint Antivirus, so that might be another avenue to be looked into...

Tried it with ESET disabled and it still crashed. Seems like Cb was the issue, thanks for the help.