The only roadblock I’ve found has been the webserial API, which enables users to flash ESP32/8266 or other devices from webpages. It’s super handy for flashing WLED or ESPHome to devices because you don’t have to install any extra software. As far as I know, it has been marked as a “wontfix” in the Firefox bugtracker as they consider it a harmful feature [0]
I mean, I understand the attractiveness, but it's a super-niche feature that wouldn't really move the needle.
The real problem, imho, is the constant glitching on Google properties like Gmail and YouTube. Even if they were really "accidental" (big if), they impact massive amounts of users in their everyday lives, and it takes very little to lose a user forever.
Maybe FF should launch a GMail competitor, and/or promote alternatives to any Google property.
I have issues, but rarely. (but I'm tech savvy... wouldn't want to tell my mom to try a different browser...)
My sense of what sites don't work properly (not necessarily 100% of the time, joy) are some big corporate, non-tech company websites, like Kaiser Permanente's and ADP's sites. My sense of recollection says these cases are frequently experienced as the combination of, "Ugh, gotta log into that stupid bloated benefits/work related site, oh and it's not working... time to try Chrome."
It’s a lot of little things. Just yesterday I was on vanguard and a select box wouldn’t display at all on FF. You start to notice these rendering differences all over the place. Gone are the days a site just flat out won’t work unless it’s specifically using an API FF doesn’t support, but it’s all the small stuff like wondering if a form is working correctly because something seems broken for FF
Also, what modern web APIs Firefox doesn't support?