Another one on the pile for no issues with Firefox. I've been using the ESR version for years and I don't think that I've ever had it crash. I'd suggest you check to see if it could be an issue with extensions, or try running it with a fresh profile.
I did have an issue with the keyboard shortcut to switch tabs (Fn+Ctrl+up/down) being taken over by macOS's window snapping so I had to figure out how to switch those shortcuts around. Otherwise, I can't think of any issue "caused by Apple". It would be up to Mozilla to put it in the store if they wanted, but I doubt that they will because they prefer to be able to manage the update cycle without having to rely on Apple or anyone else.
I did have an issue with the keyboard shortcut to switch tabs (Fn+Ctrl+up/down) being taken over by macOS's window snapping so I had to figure out how to switch those shortcuts around. Otherwise, I can't think of any issue "caused by Apple". It would be up to Mozilla to put it in the store if they wanted, but I doubt that they will because they prefer to be able to manage the update cycle without having to rely on Apple or anyone else.