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.