Firefox 3.5

Mozilla Firefox 3.5 is a version of the Firefox web browser released in June 2009, adding a variety of new features to Firefox. Version 3.5 was touted as being twice as fast as 3.0 (due its TraceMonkey JavaScript engine and rendering improvements). It includes private browsing, has tear-off tabs, and uses the Gecko 1.9.1 engine. It was codenamed Shiretoko during development, and was initially numbered Firefox 3.1 before Mozilla developers decided to change the version to 3.5, to reflect the inclusion of a significantly greater scope of changes than were originally planned. It is the last major version to support X BitMap images.

Estimates of Firefox 3.5's global market share in February 2010 were around 15–20% and rose rapidly in July 2009 as users migrated from Firefox 3.0. From January 2010 it began to decline as users migrated to Firefox 3.6. Between mid-December 2009 and February 2010, Firefox 3.5 was the most popular browser (when counting individual browser versions) according to StatCounter, and as of February 2010 was one of the top 3 browser versions according to Net Applications. Both milestones involved passing Internet Explorer 7, which previously held the No. 1 and No. 3 spots in popularity according to StatCounter and Net Applications, respectively.

Due to the January 2010, well-publicized vulnerability in Microsoft's Internet Explorer browser, the German, French, and Australian governments had publicly issued warnings to Internet Explorer users to use alternative browsers, at least until a fix for the security hole was made. The first browser they recommended was Mozilla Firefox, followed by Google Chrome.