I've been mostly very happy with Firefox - I tend to forget that I am using it, which is a good sign. Except that every time I clicked on a link in Office, or tried to open a URL from my Address toolbar, I got 2 windows, or 2 tabs, of the same page. Or I'd get an error message about not finding the executable.
Luke's post fixed all that:
public virtual MemoryStream: More problems with Firefox