TG Daily has some info about what the Mozilla developers are working on for Firefox 4:
One of the new features is called "Prism," a next-gen software technology that would allow users to run Web-based content offline. For example, users would be able to visit websites and still access non-dynamic content even when they don't have an active Internet connection.
The other big feature already unveiled for Firefox 4 is called "Weave." This is an option that would basically create a set of online preferences and settings that is stored for each user instead of each computer.
In other words, users would be able to bring up their bookmarks, font settings and privacy preferences no matter which computer they use. This feature is set to be in its first stages in Firefox 3, but Firefox 4 will reportedly allow users to access these profiles on other browsers as well.