A simple temporary fix is to go in to "about:config" and change xpinstall.signatures.required to false (and then restart FF for completeness)
That hasn't helped, my plugins are still disabled. I can't use my (payware) Internet Download Manager because the integration plugin has been disabled. I can no longer see screenshots in forums either, all pictures are blank (fine in IE). Firefox is basically broken.
I'm off to some other browser and never coming back.
After 15 years or more using Firefox I am now so fed up of this kind of stuff, I too am ditching it. I know this particular incident is an 'accident', but already, because of the changes that Mozilla keep making, I still have to use v56 - most of the plugins I have come to rely on won't work with later versions.
And stuff is always being shifted around. Why on earth for example after so many years has the Home Page icon moved from the far right to the far left of the browser? After so long using FF, these things become second nature.. when they get moved, (especially when they are buried in a menu somewhere) hunting them down and 'relearning' where Mozilla have put stuff is really irritating.
This glitch is the last straw for one who is not in the first flush of youth any longer and has limited patience with this kind of stuff!
So, sorry: bye bye Firefox...