I'm viewing AtariAge right now on a 600MHz Pentium III, using the newly-released Firefox 5. It actually works surprisingly well even on such old hardware; I've noticed less slowdown and a much smaller memory footprint compared to previous versions.
I was surprised, too. But yes, Firefox 5 was just released. If you're running Firefox 4, click on the "Help" menu, select "About Firefox", and it will download and install the update (about 8.5MB) automatically. So far, it looks and feels just like Firefox 4, only smaller and lighter.