Try this one then.
It worked for me, but needs to be done each time you run Firefox.
If you have the regular build (not the nightly/dev one) then the xpinstall.signatures.required set to false doesn't fix this. But I got all of my add-ons back by going to about
:debugging. Check the box to enable add-on debugging. Then click Load Temporary Add-On. Browse to your Firefox profile in appdata:
Code:
C:\Users\<Username>\AppData\Roaming\Mozilla\Firefox\Profiles\<some random ****>.default\extensions
In the extensions folder there are .xpi files, those are your extensions that you had. Load each one and don't close Firefox until they fix this.