I don’t even think that’s remotely true.
I’ve seen two cases that actually directly impacted my ability to use Firefox. I can only presume there are many more. Those being supporting the column-span CSS property (available since 2010 in other browsers with vendor prefix, and early 2016 without, while being late 2019 for FF) and supporting iPad OS’s multi-window functionality (introduced mid 2019, Firefox has had it for just a handful of months now). I have first hand experience telling me very directly that this is true.
There’s also been a lot of talk about Firefox’s lack of support for PWAs. I’ve not experienced that myself to be able to comment more than to say I’ve noted others have complaints.
The vast majority of my time on Reddit was on smaller communities where that wasn’t true. The experience on Reddit of the defaults compared to more niche communities was like night and day.