God damn you, Mozilla. You DO NOT do a major, massive update to people's web browsers without any warning in the middle of the work week! This is Change Management 101, go buy a bloody book on the subject!
And don't you go telling me that it's in the media/news/whatever. If you are going to screw with people's machines this way, you tell the up front and DURING THE INSTALLATION PROCESS that big changes are coming. You DON'T spring it on them as just another update.
Anyway, if you are still reading this, possibly you are like me and not too enthused with Quantum breaking all your extensions, add-ons or whatever. You could waste ages trying to figure out if Quantum supports everything, plus dicking with the UI. Or you could roll back.
Fortunately, rolling back is indeed simple: all you have to do is close Firefox and re-run the Firefox 56.0.2 installer again. Yes, it really is that easy.
More detailed instructions, including what to do if the process goes awry, can be found here at SuperUser. But that's really it.
- You need a copy of the Firefox 56.0.2 installer. Mozilla makes this harder to find than it should be, so I don't have a link handy. It's out there.
- You should back up your profile directory before downgrading. This is as simple as copying %APPDATA%\Mozilla\Firefox\Profiles\ to another location for safekeeping.
In my case, everything was perfectly preserved after the downgrade except for the theme (which I had changed in Quantum) and a new spacer in my toolbar setup, both of which were trivial to fix. Good luck.