In the last 5 years I’ve run across maybe 1 site that didn’t work properly in Firefox. And another that MIGHT not have worked right, but I was only guessing it was related to FF.
However, since FF dropped PWA support I do use Chrome for a handful of sites that either are PWAs or you can use Chrome’s open as application feature, which is real nice for a few things. Is that what you mean by “Web Apps”?