The latest build also sported the first attempt at accelerating Firefox 4's page rendering by tapping the graphics processor. The hardware acceleration, available only in Windows Vista and Windows 7, is disabled by default.
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Firefox 4 Beta 4's most visible addition is "Panorama," a new name for what Mozilla had been calling "Tab Candy." Largely driven by the work of Aza Raskin, creative lead of Firefox, Panorama lets users collect tabs into sets, graphically displays those sets, and when users open a tab, shows only those tabs within the group.
Mozilla, which calls Panorama a tab manager, has argued that it's the next step in the evolution of tabs.
Firefox's rivals have nothing like Panorama. Apple, for example, introduced "Top Sites" to Safari last year, while Google's Chrome has had a similar "Most Visited" feature since it launched in 2008. But both simply graphically represent frequently visited sites using thumbnails.
Microsoft has not revealed the user interface or how it will use tabs in its next browser, Internet Explorer 9 (IE9), which is scheduled to ship as a beta on Sept 15.
Firefox Sync, the other major feature new to Beta 4, is not new to Mozilla: The service that keeps bookmarks, passwords, browser history, open tabs and other data consistent across multiple computers and devices traces its roots to 2007 and a project then dubbed Weave.
Sync has been available to users of earlier Firefox editions through an add-on and to iPhone owners via the free Firefox Home app, but this is the first time that the functionality has been baked into the browser.
Chrome and Opera have had integrated synchronization since 2008.
Mozilla also debuted hardware acceleration in Beta 4, but left the Windows-only feature turned off. To switch it on, users must edit the browser's "about:config" file using instructions Mozilla has posted on its site.
Firefox 4, like IE9, relies on Windows' Direct2D API (application programming interface) to boost rendering speeds by shifting some chores from the computer's central processor to the graphics processor.