I'm not sure what the difference is between sites "not working" and "breaking", but as a counter point, I've seen neither in the past three or four years of exclusively using Firefox as my web browser.
back
2 comments
As yet another counter point, I haven't had any problems with any site either. Actually, I think I haven't had any real problems with any site in Firefox since the IE6 days.
Been using Firefox forever, since it was called Firebird, migrated to it from Netscape at some point.
I use Firefox almost exclusively.
One example of a page that doesn't work is Amazon's "Today's Deals"[1] page.
[2] is screenshots of the same page in Firefox (top) and Chrome (bottom). I do have uBlock Origin installed but disabled for Amazon.
That page works fine in my FireFox (v81 on Windows, also with uBlock Origin enabled). Have you checked the behavior with a clean profile?