Thank you for offering to help! I actually just figured out how to fix it right now myself. The solution is to delete the following lines in prefs.js, then restart unbranded-Firefox twice:
The first start takes longer, so I can tell it's resetting something internally (not sure what). However, the second restart is what makes it start working. Not sure why.
Edit:
For anyone else trying to auto-obtain the latest stable Firefox nightly, here's a hacky command (change for your OS as needed):
Glad to hear you worked it out, good luck! Looking around there are a few levels of cache around whether addons are enabled, I guess those are what you need to clear it.
It's a temporary hack, but you might also want to look at temporarily loading extensions with about:debugging, this works even in official release.
Edit:
For anyone else trying to auto-obtain the latest stable Firefox nightly, here's a hacky command (change for your OS as needed):