Looking back, we are technically at approximately 4.1.3 (or 6, depending on lower digit verison numbering) with firefox 9 being 4.2.
Firefox 5 added the CSS 3 / HTML 5 standards into the browser in full implementation, so thats a big deal. Firefox 9 adds type inference like V8 has that makes Jagermonkey competitive with V8 again, so thats a big deal. 6 - 8 just did some light UI tweaks and optimizations, so nothing important there.
The biggest downfall about the rapid release ideology is you can no longer hype a version. You can't say Firefox X will be a big deal because its only 3 months since X - 1 came out and 3 months until X + 1. It hurts products more than helps them, especially at the corporate level, where they don't rapidly update browsers and running Firefox 4 when Firefox 8 is coming out and Firefox 10 is the currently alpha drives sysadmins insane because they have no idea the difference in standards and security between any two versions, whereas a jump from something like 3.X to 4.X meant a big deal.
Now there are no big deals, and that, in my opinion, hurts the product.