firefox.com has a relatively low score because of a goal to keep it accessible by older browsers on older operating systems (i.e., out-of-the-box, you should be

firefox.com has a relatively low score because of a goal to keep it accessible by older browsers on older operating systems (i.e., out-of-the-box, you should be able to load it on the oldest systems Firefox supports, which I believe include XP SP2).