According to an official roadmap of Mozilla foundation, Firefox 4 is scheduled to be released in 3rd of 4th quarter of year 2010. Mozilla has also planned to release Firefox versions 3.6 and 3.7 in 2009.

According to the roadmap, Firefox 4 will be launched with many major additional features, with a major change in it user interface, other changes that will be included in the version are process separation for chrome and content, new extensibility mechanism through API’s, platform enhancements, end user features enhancements and speed and responsiveness improvements.

Mozilla foundation has also planned to release version 2 of its mobile browser Fennec along with its browser Firefox 4.

Firefox’s version 3.6 will be released around Q3 or Q4 in 2009, it will be with some improvements in speed and responsiveness, other enhancements include integration with Windows 7 and end user feature enhancements. Mozilla is working hard on its browser to provide ‘support delivery of mobile devices and Windows CE’ and also to stabilize update Firefox quickly to improve application responsiveness.

Version 3.7 of Firefox will have enhancements like running plugins in a separate process to improve overall application stability and responsiveness, improve setup time, platform enhancements, end user feature enhancements, and other feature as well.

For details of features to be included in these new releases, you can visit: https://wiki.mozilla.org/Firefox/Roadmap