Mozilla's latest roadmap shows Firefox 4.0 is scheduled for October or November 2010. The new browser will feature a new user interface, which is said to be more like Google Chrome with a design that integrates Windows 7 graphics features. More info at TG Daily.
The current release schedule is just in draft status, which means we have to take it with a grain of salt. But the guys over at Mozilla are surely busy getting two new versions of Firefox out by mid 2010 and apparently a completely new browser by the end of 2010.