In its emulation of Google Chrome's quick release schedule, version 7 of Firefox has already appeared in Mozilla's Aurora incubation channel, while version 6 has moved to beta.
Neither version brings the kind of major changes we've become used to in full version number upgrades of Firefox, but they do add welcome performance boosts, standards support, and minor interface and functional tweaks.
Under Mozilla's new Rapid Release calendar, new versions of Firefox will come out every six weeks; this way you'll know it's time for a haircut when a new Firefox version appears. Firefox 6 is scheduled to be ready for release on August 16, while version 7 will move to the beta channel on the same day. Version 7 then moves to the release channel on September 27, at which time Version 8 goes to beta. You get the picture.
Note that the release dates pertain to Firefox for Android as well to the Windows, Mac, and Linux desktop versions, though the mobile browser gets different features than the desktop version in each release.
No, you aren't crazy, Firefox 4 did just launch a week ago.
Mozilla is borrowing a page from Google Chrome and speeding up the development cycle for Firefox releases, setting new iterations of the browser for fixed time periods and bulldozing over features that just aren't ready to make it into a new browser release.
And if Mozilla sticks by its newly proposed plan, that means that we'll be seeing Firefox 5 on June 21—following a shortened 13-week development cycle instead of the proposed 18-week cycle for all future Firefox builds.
Within this 18-week cycle comes a new development stage that adds on to Mozilla's three previous update channels: Nightly, or builds created from the mozilla-central-repository that are highly unstable, but incorporate the latest texts and fixes; Beta, which ups the quality demands of features and tweaks added via the nightly builds; and Release, which becomes the version of Firefox that most consumers are used to using.
Mozilla's new stage, Aurora, will be a nightly update that splits the difference between the chaos of the company's Mozilla-central build (or Nightly build), and its Beta build.