Only a short time ago, we had to wait over a year for a major new release of any web browser. Often longer. There was a huge gap between Firefox 3 and Firefox 4, with many alphas, betas and numerous release candidates. That was before Google changed the game with Chrome, with rapid releases, quickly switching between stable, beta, dev and Canary. Recently, Mozilla decided to change the way they would release a new version of Firefox, somewhat mimicking the Chrome release schedule. You can now download a stable version of Firefox, currently v5, Aurora which will quickly become v6 beta and even a Nightly, which is currently v7 alpha.
Firefox 5 is now the stable final release and a relatively minor update to Mozilla's flagship browser, though still definitely worth the download.
Once installed you'll find it difficult to spot any visible changes. The Do Not Track privacy feature introduced in Firefox 4 has been relocated so it's a little easier to spot (Tools > Options > Privacy, where it really should have been in the first place) but that's about it.
For more detailed information, view the Firefox 5 release notes.
This is the portable edition of Firefox 5.