I didn't have websites outright breaking on FF like solardev, but I had other issues with it, like cloudflare getting me into an infinite loop of asking me if I'm human and not letting me browse the website. This happened even if I deleted my firefox profile and started fresh with no extension. Note that this behavior of Cloudflare is highly dependent on the settings of their anti-ddos stuff, some websites have it set on a higher level of defensive behavior than others, I didn't have this issue everywhere.

This wasn't due to my computer's IP, the problem went away the moment I browsed the same website with Chrome, and this time I wasn't even asked to click the checkbox to prove that I'm human.

Sadly I don't remember anymore. I ran into plenty of them 3 or 4 years ago (every week or so), then I stopped using Firefox altogether because of that. At work, some coworkers were using Firefox last year and it broke various third party libraries (especially graphics heavy ones). Rather than trying to fix the bugs, we just decided to deprecate Firefox support altogether because its usage was down to like 2 or 3 percent. Those resources were better spent on, say, improving mobile usability and performance for everyone.

Edit: I wish Mozilla would just fork Chromium and add/delete whatever privacy things they want, like Brave. There's no reason to maintain Gecko anymore.