I believe that my updated graphics driver is the cause of this "some-firefox-buttons-not-working" issue (https://support.mozilla.org/en-US/questions/1194760). How do I find it (what's it called in update history?) and revert it?

By the way, I also can't access "driver manager" without screen freezing up and requiring a restart. I suspect that goes under the same "graphics driver issue" as well. Tested different kernels, didn't help.

I'm hesitant to reverting system with "timeshift" because I do not know how many weeks have gone since the graphics driver was updated.

Are we speaking of Mint 19?

As far as I'm aware, rolling back will only affect system programs/settings - provided you didn't ask it to back up additional data.

I would make sure you have a current backup of your user data on external media so that if the worst happens, you could install from media and recreate your user data.

Reinstalling is a lot of work as you should also take copies of certain system files e.g.
/etc/fstab together with a list of partitions/filesystems & their approximate sizes;
A list of the packages you have installed;
The contents of any crontabs;
A listing of /etc/passwd, so that you can create users in the appropriate order
UID=1000 = first user,
UID=1001 = second user, etc.

I'd think carefully before proceeding.
If you are able to restore your PC, then you can try some riskier options.

But first - why not try Mint19(?) in a VM - Does Firefox exhibit the same behaviour?

I'm running Mint 18.3 KDE and the solution was found with the help of this link: . System settings->display and monitor->Compositor -> change backend to Xrender (from OpenGL 2.0). I do wonder though why changing KDE rendering backend to xrender solves it for me, but using OpenGL 2.0, or 3.1 for that matter, doesn't.