What websites? I use Firefox as my daily driver on desktop and mobile, and I rarely run into problems. Like so infrequently that I don't even remember the last time.
Some websites display warnings even though everything works fine, like web.skype.com.
But that's the closest thing to doesn't work I've ever seen on Firefox.
I was curious of what their menu looked like as I have one by my work. Haven't checked it on desktop, but on Android, the menu items never get loaded. ($10-12 for an á la carte chicken sandwich from a fast casual place is a ripoff anyway)
Edit: It looks like it's the mobile site/formatting that is broken. Using desktop mode lets the menu items come onto screen. (Firefox 126.01 on a pixel8pro with Android 14) The same issue seems to also be present with chrome under my work account.
I still argue that they are present, as I had to with it doing taxes a couple months ago. (Just not going to give those sites away)
No problems loading that page on Firefox for Android or desktop for me. Are you using Firefox or a fork of Firefox? Do you have any extensions or about:config changes that may be affecting the page rendering?
I encourage those in this situation to do their small, small part in fighting for the future of the open web by only switching to Chrome when necessary.
Last I tried, I had serious issues using it on Comcast's billing pages and Quest Diagnostic's site, among others. The pages would not load at all until I went to Chrome.