I just assumed it was one of their almost daily “improvements” that periodically breaks the browser extensions until the providers can design workarounds. (My email links still default to Firefox, so I noticed the issue last night).
Firefox has been on a suicide run for some time now; I suspect Google folks are behind it to shift everyone over to a flavor of Chrome (Brave for me, Chrome in a pretty dress).
Choices are rapidly narrowing to Google and Microsoft.
I think there is a branch of the old Firefox out there. I’ve got a very old Seamonkey so I’m not sure where they have gone recently.
a temporary fix is to go to “about:config”, search for “xpinstall.signatures.required” and set it to False. install your adblockers again or whatever you need- then go back into go to “about:config”, search for “xpinstall.signatures.required” and set it to False. and set it back to true
Trying it now- it seems to be working,