It hurt to deprecate because I used to be such a Firefox fanboy. The last few years there's been a compounding effect of Chromium/Blink adding features that aren't supported elsewhere and Firefox just having a slew of very outdated bugs that haven't been fixed for years that have made it difficult to use. The percentage of our users who use Firefox has also steadily declined... most likely because we have to throw up "Unsupported browser, things may not work" warnings all over the place because the number of random bugs users keep running into.
I'm sure Firefox works fine for most sites, but if you're one of those that it has an issue with, well... just like IE, below a certain % of usage share, we just have to make the decision to not support it.
Actually, this was a learning experience that caused one of our devs to switch from Firefox to Chrome too. We didn't realize how far behind Firefox had gotten until we had to debug the issue and read through their tracker. Mozilla just ignored the threads after a while and they kept building and building.
We haven't put up a warning banner yet but probably should. It's like IE all over again lol.
Firefox was less than 3% of our user base, even less than Edge, and we just didn't have the resources to keep debugging for one browser. Everything else is Blink and Webkit.
RIP...