find that hard to believe. but even if you find something using an api not implement by firefox, chances are you definitely do not want that feature anyway, the firefox gave in to really awful stuff and only drew the line on obviously egregious privacy violation ones.
I definitely run into pages broken in firefox desktop or especially firefox mobile. Extra especially on proofs of concept advertised here on HN.
Many see 'it works on Chrome and mobile Safari' as 'it works' and they can get project signoff / ship / get paid / whatever and don't care about other users
The company that has the application may not know until a few users complain (if they complain) and by that point it could be too late due to the contract, or they may not understand what a different browser is or care either.