There is a discussion on Hacker News, but feel free to comment here as well.
What sites are these? I’ve not had this in years and when there is problems the site normally says unsupported browser rather than leaving me to guess
Other than YouTube, which purposefully slows Firefox down, I only ever encountered one webpage in years where the issue was 100% “caused” by my browser choice – the left pane (lesson list) in my school’s tutoring platform would not scroll due to what I assume was a trivial HTML oversight. After reporting the issue, they insisted that they would not support browsers other than ones based on Chromium and Safari and tried to convince me to switch. I don’t have access to the site anymore and I did not understand web development at all back then so I could not create a patch myself; I just worked around the issue using a very tall, zoomed-out “mobile view” to reach the off-screen buttons.