According to the Firefox official roadmap, Mozilla wants to ship Firefox versions 4-7 this year. The primary focus of Firefox this year is to make sure that there the time between any user interaction and feedback from the browser is less than 50 milliseconds. Below is a list of features that each version would have:
Firefox 5: Absorb the Account Manager and F1 Simple Sharing.
Firefox 7: Split everything into separate processes and change to XBL support.
[Source: Download Squad]