There’s plenty of weird cruft in Firefox’s ~30 year old codebase that causes bugs and unspecified behavior. For example, Firefox’s ContentEditable code deletes things “backwards” compared to all other browsers and operating systems (bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1735608) - this behavior isn’t standardized in any spec, and Firefox is different from other browsers.

There are plenty of gotchas in layout/rendering as well, where either the standard is under specified, or Firefox has some small bugs. Maybe Chrome has many-chrome only APIs, but the developer will always need to test in Chrome and iOS Safari 13 (or whatever your oldest supported iOS version is).

Chrome and iOS are where the users are, and a good website or app should be usable and beautiful for everyone.