Trezor is... a crypto hardware wallet? I can't imagine this sort of thing is popular enough to block firefox from mainstream adoption.
The pixel repair site is an interesting example, but AFAIK, Pixel phones have about 2% of the US market share and this website is only useful to a small number of those users (those who encounter a serious error) and isn't something even those users would be using often. If I had to use this website and couldn't get my phone fixed another way, I could install and use Chrome (or I presume another Chromium browser) and use it for this single purpose. It would take me only a few minutes/hours(?) and then I'd be done with it. Why would a site like this keep me using Chrome permanently? I just can't imagine that.
I think this is pretty typical of these niche web features. Like most users have no need for web midi and most likely don't even own a midi device that it would work with. But niche websites can find great value in the api being available and offer sites to users which bring them value without having them install a native app. Also I'm not sure if it's a good idea to give people a reason to install and use a competitors browser.
I don't think either of these are examples of websites that might plausibly be holding Firefox back from mainstream adoption. I was expecting something like "[popular feature] of facebook or youtube doesn't work"
I'm sure I'm not the only person who has Chrome installed as a backup, just in case I run across a site that doesn't work right in Firefox, or if I just suspect such a problem exists. Having an alternate browser installed is trivial; it's not like using a different OS.
The pixel repair site is an interesting example, but AFAIK, Pixel phones have about 2% of the US market share and this website is only useful to a small number of those users (those who encounter a serious error) and isn't something even those users would be using often. If I had to use this website and couldn't get my phone fixed another way, I could install and use Chrome (or I presume another Chromium browser) and use it for this single purpose. It would take me only a few minutes/hours(?) and then I'd be done with it. Why would a site like this keep me using Chrome permanently? I just can't imagine that.