High Quality Content by WIKIPEDIA articles! 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.