[10] The primary goals for this version included improvements in performance, standards support, and user interface.

[11] There was one security update in April 2011 (4.0.1) and version 4 of the browser was made obsolete by the release of Firefox 5 in June 2011.

[15] Early mockups of the new interface on Windows,[16] Mac OS X,[17] and Linux[18] were first made available in July 2009.

New features included improved "doorhanger" notifications, Firefox Panorama (a feature that lets the user visually group tabs),[19][20] application tabs, a redesigned extension manager,[21] Jetpack extensions support, integration with Firefox Sync,[22] and support for multitouch displays.

Firefox 4 is based on the Gecko 2.0 engine, which adds and improves support for HTML5,[25] CSS3, WebM, and WebGL.

It improves performance by compiling "non-traceable" JavaScript into machine language for faster execution.

The browser has made significant progress in Sunspider JavaScript tests as well as improvements in supporting HTML5.

[32] Using hardware acceleration allows the browser to tap into the computer's graphics processing unit, lifting the burden from the CPU and speeding up the display of web pages.

On 22 March 2011, and during the 24-hour launch period, Firefox 4 received 7.1 million downloads, as counted and verified by the Mozilla Foundation.

[53] Before that date, 3 million people downloaded the second release candidate of the browser, which later became the final version.

Second-day downloads for the browser were reported to be 8.75 million, but the lack of an official representative from Guinness to monitor the numbers, made the record attained by Firefox 3 only unofficially been broken.

[53] On the official launch date, the usage share for the Firefox 4 was 1.95%, which was 0.34% higher than the previous day according to analytics website StatCounter.

On March 26, 2011, Firefox 4's usage share exceeded that of the 10-year-old and discontinued Internet Explorer 6 for the first time.

Firefox 4 represents a departure in user interface layout and behaviour from previous versions.

In beta 7 introduced new config option to limit the number of tabs loaded at once during session restore.

This also made possible to lazy load tabs,[64] the preferences option to switch this behavior appeared in version 8.