Version 3.5 was touted as being twice as fast as 3.0 (due its TraceMonkey JavaScript engine and rendering improvements).

It was codenamed Shiretoko during development,[6] 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.

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.

[21] It included a new TraceMonkey JavaScript engine, which is not enabled by default, and the implementation of the W3C Geolocation API.

[23] The new tab switching behavior and interface was dropped at this beta, since the developer found it needed more work.

These include support for the

[34][35] The goal is to offer video and audio playback without being encumbered by patent issues associated with most plugin and codec technologies.

[39] A minor change for Version 3.5 is the default search engine in Russian language builds, which uses the search engine Yandex rather than Google, after a survey of Russian Firefox users indicated they preferred Yandex.

Starting July 14, 2009, the upgrade to 3.5 was offered to users of Firefox 3.0 through the automatic internal "push" update mechanism.