Unfortunately, while Firefox has made significant leaps the last couple of years, it's still not a viable alternative to Safari for me. It doesn't feel particularly native (neither the "chrome" nor form controls etc.), and the UX is still clunky compared to native Mac-first apps.
One particular way in which Safari absolutely beats Firefox is the address bar. Most of the autocompletions that Safari provide are exactly what I want. As an example, if I type "mapq" into Safari, it suggests mapquest.com (a site I've never visited before, so this is not based on anything from my history). For Firefox, it suggests a Googles search for "mapquest", which is just stupid. Aside from suggesting web sites, Safari is superb at showing good results (Wikipedia is often a top hit), and it is awesome at providing autocompletions from my bookmarks and history.
Firefox also seems a bit lost when it comes to innovating, and keeps coming up with weak concepts that don't get any tractions; Personas comes to mind. Container tabs are a similarly interesting idea with a weird implementation that requires you to micromanage your tabs, which I certainly don't want to, and provides a "techie" solution to something that should just be invisible and default (i.e. all web sites should be "contained").