Those experienced with web dev around here will tell you Chrome is the main (or only) browser tested with most websites these days. Some things might be slow or broken in Firefox. One example situation that comes to mind is you can't scroll to the bottom of the page on some sites in Firefox due to terrible spaghetti layout design, so the submit button is not normally reachable. Or it loads under a banner and becomes unclickable.

There's also some sites that seem to actively make things worse on purpose or refuse to load even if they otherwise do work. I think YouTube was noted for doing this a few years ago.

In other areas, small company sites may claim Firefox just doesn't work on their site. Sometimes prompted because an ancient version once was broken and the browser entry list was never updated, or they simply forgot to account for it in the grouping of "Internet Explorer or Other." I see this more often than I would like.