That may not sound like a dramatic overhaul for a whole new version number, and indeed it isn't. Mozilla is following in Google's Chrome footsteps with a faster release cycle that means new versions arrive more rapidly but the differences from their predecessors are less dramatic. It also means a major change in version number doesn't mean a major overhaul has taken place.
Mozilla has three main versions of Firefox now: the release version for mainstream users, the beta version with new features under testing, and the most raw, Aurora, a version to introduce those new features. Those three versions correspond to Chrome's stable, beta, and dev releases. There's also a nightly Firefox build for those who want to try whatever patches have arrived in the last 24 hours, but it's most likely to be unstable.
The new process means that whatever is done on a particular schedule can ship in the new version--a more calendar-focused process than feature-focused process. One goal of the approach is to reduce the penalty of missing a train, because with a rapid release cycle, another train should come to the station soon.
Another feature in the new beta is the ability to switch among the various versions of Firefox by clicking "change" in the "About Firefox" dialog box.
Also arriving yesterday was a new beta of Firefox 4 for mobile devices, a version that adds support for a do-not-track technology Mozilla hopes Web sites will embrace to aid those who don't want their personal behavior or data recorded.
Mozilla also is experimenting with another idea that's all the rage, a minimalist user interface frame. Firefox 4 did away with some items, such as its menu bar, but an experiment called LessChrome HD does away with just about everything except tabs. (Chrome in this case refers to the user interface items, not Google's rival browser.) With LessChrome HD, user interface elements such as the address bar pop up when the mouse hovers in the appropriate location.
Chrome, which has long embodied the minimal-UI ethos, has a similar option called compact navigation under way that makes the address bar part of the tab, too, so that a few more precious pixels of Web page are visible. This cnav option is only available on Windows right now and isn't enabled by default.
With Web applications, it's important to show as much as possible--especially since those apps can come with their own user interfaces, such as navigation and menu bars. It's not yet clear yet just how much of the browser interface should be hidden, though; there's still plenty to do with a browser beyond clicking its tabs, and hiding the address bar by default would doubtless cause another round of griping from those who didn't like their menu bars and status bars disappearing, either.
Download Firefox Bete Version 5.0