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:

  user_pref("extensions.databaseSchema", 27);
  user_pref("extensions.e10s.rollout.policy", "50allmpc");

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):

  curl -s "https://wiki.mozilla.org/Add-ons/Extension_Signing" | sed -n "0,/Windows 64-bit Installer/{s/.*href=\"\\(https:[^\"]*\\.zip\\)\">Windows 64-bit Installer<.*/\\1/p}"

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.

Yeah the temporary thing was already working in the official release, but it's temporary so I was looking for the normal way. Thanks! :)