Comment by bityard

It's crazy, but in 2026, websites still check the user agent string and will simply refuse to work if it's not one that they like. Financial and enterprise software is the worst for this. It's one of the reasons Vivaldi switched to simply copying the Chrome user agent string instead of their own.

Some sites also simply to not test their stuff on Firefox since it has such a small market share, and Firefox _does_ have minor incompatibilities that only tend to show up when using overly fancy Javascript or CSS frameworks. (But this is far less common than the first point above.)