Well that was fun. I upgraded from Mint 18.3 to Mint 19 because ... reasons. One of the first things I ran into was that Firefox wouldn't load my profile. The error was that it was created in a later version of Firefox than what I was running now. Whaat?
Turns out that the profile tracks not just the major/minor version but the specific builds have a timestamp suffix, and the one for Mint 19 was built first, so appeared to be an earlier version. Editing ~/.mozilla/firefox/[profile]/compatibility.ini to tweak the timestamp (making a new profile seems like the easiest way to get one) fixed it.
What's broken next...
@SetecAstronomy I probably would have spent three hours trying to upgrade FF without breaking everything adjacent
@zpojqwfejwfhiunz Near as I could tell, it was already the latest. At least the latest in the repos. I got a hint to my solution from someone facing the same issue with an Ubuntu upgrade some time back. I didn't end up using any of the solutions in the thread, but it did point me to the troublesome file where I was able to determine it was a version difference of ... a few minutes and nothing more. At that point I backed the whole profile up and just swapped in the "correct" version and tada, one less thing is broken.
Fun fact: The reason for the OS upgrade was not fixed by the upgrade. It looked like it was going to be, but apparently I wasn't quite thorough enough in my investigation before hand.
@SetecAstronomy that is NOT a fun fact!
A bunch of technomancers in the fediverse. Keep it fairly clean please. This arcology is for all who wash up upon it's digital shore.