For whatever reason car manufacturers are the worst at it. Microsoft is also pretty bad (worse than Google).
* myADP had some kind of bug preventing firefox authentication that lasted months.
* Elan financial redid their credit card site for fidelity visa and had a CSS bug that hid the card total balance. This lasted for about a month.
* bitbucket (work required) had this bug that prevented pages rendering in firefox. It was some kind of JS client side bug and it impacted a bunch of people in my company. After months of support request they gave us some console commands that opted into an experimental feature which resolved the issue. This one was especially frustrating as they kept telling us to just clear our cookies and delete our profiles.
* Some random IRS system I had to use for either requesting a tax transcript or validating my identity just didn’t work.
It’s actually been so frustrating my coworkers joke anytime there is a bug I’m experiencing that it must be another Firefox only bug.
I'm not saying that to dismiss the problem, I believe GP that it's crashing; more bringing it up to say "there is something going wrong here that's not at the browser/site level that could potentially be fixable for you on your local device if you want to use Fastmail in Firefox."
For years they had a terrible implementation that only worked in _old_ versions of IE. They had no incentive to change because they were the main players in that space. I loathed them because their tagline was "Anytime, anywhere" which was clearly missing an asterisk.
Between blocking tracking, with DuckDuckGos privacy plugin, and using Firefox I'd say that blocking tracking breaks WAAAAY more sites.
It fascinates me how many people I know have uBlock (Origin) installed in Chrome and see nowhere near as many whiny banners and "stop running an ad blocker" nonsense as I see running Firefox without an ad blocker just ETP and MAC. The people complaining "Firefox is an ad blocker" are making it clear that they don't actually care that you are blocking ads or not, they care that you are blocking trackers and hate you for trying to maintain a semblance of privacy. Keeps saying the quiet parts out loud.
Most of the sites that do that are a "close tab and ignore" thing, some are a sequester to their own account container and feed them garbage (create a dumb account for them, turn off ETP just for them just in that container). It is still only a once-a-month case where such a site I need to get something done on is entirely broken in Firefox and I need to open it in something Chromium based, but that's still too often and getting worse.
But that "Firefox is an ad blocker, please use a less private browser" is a daily annoyance for me for sure.
I couldn't apply to become a registered engineer in my state using Firefox because the form was in an iframe, and for some reason the iframe didn't render on FF.
The credit portal for Shell Fuels only works in Chrome and doesn't display at all in FF due to a console error. (Enough of the page renders to make you think that the page is complete but i.e. the buttons to apply or make a payment don't render).
The field for setting the date of a consultation (work) in our accounting package doesn't work in FF, and no matter how many times I write to their support team they just write back with "Firefox is not supported, please download Chrome". There was a similar attitude at a software company I worked prior, but I managed to wear them down and get Edge (modern), Firefox and Safari on the QA supported browser list with the reasoning that they are the default browsers for Windows, Linux and macOS respectively.
But come on, browser standardization is so good these days I honestly don't understand how people are making pages that only work in Chrome?
I use FF by default.