Only once have I experienced that something I'd actually write for production only worked in FF and not across all modern browsers.
Protip for anyone who reads this and thinks "but my employer doesn't care about Firefox':
I typically used FF also while testing other peoples work and if it didn't work in Firefox then 9 out of 10 times it didn't work in any other browser than Chrome (had some devs on my team who really didn't seem to care about cross browser compability but it would work in most browsers before it was approved :-))
Only once have I experienced that something I'd actually write for production only worked in FF and not across all modern browsers.
Protip for anyone who reads this and thinks "but my employer doesn't care about Firefox':
I typically used FF also while testing other peoples work and if it didn't work in Firefox then 9 out of 10 times it didn't work in any other browser than Chrome (had some devs on my team who really didn't seem to care about cross browser compability but it would work in most browsers before it was approved :-))