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.