Add this to the peculiarities: I cannot determine whether it is OS, rascal or Firefox origin:
One of the first things done with a Firefox update is a number of JavaScript files are edited to remove unwanted items.
Some of these files are located in /usr/lib/firefox and /usr/lib/firefox add-ons.
Also files in the profile are edited to maintain current settings and remove unwanted items.
Areas of edit are sync, search, mail, extensions and some other tweaks.
Found: On cold boot the files in the /usr/lib/firefox and /usr/lib/firefox revert to the installed default.
This is after a previous session where the files were left in the edited state with backup files reading the same.
This occurs in both the 19.3 and 20.1 versions of the Linux Mint Mate OS.
This has been tested and double checked.
If left in the default state it defaults the files in the profile possibly negating various extensions.