It’s rarer than is used to be but I know Ive had issues in the past with firefox on some insurance websites, some banking websites, I know Ive seen sites where they block you from even entering but if you change the user agent it works. It’s less common than it was 5 years ago though as there is a report website feature and they work hard to try and fix compatibility.

It wasnt firefox’s fault though more the web developer. I dont have specific examples because I didnt save them and it’s been a while. Its not as bad as during the ie6 days of the internet though. Now that was dark times.

Edit: Also sometimes it’s addons and people mistake that for the browser and dont tweak their addons before blaming firefox.