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.
Yeah I’ve definitely run into issues where case sensitivity causes problems. Especially in programs that are cross-functional between Windows and Linux. Like when I recently downloaded some bios files for a Playstation emulator and I spend time figuring out and troubleshooting why they weren’t working until it finally hit me the door McFly it’s cause the file name was in lowercase not uppercase. Than I cared to admit to figure out