Mozilla has created a draft document about the goals for Firefox 4.0. This browser is codenamed Firefox 3.6 "Namoroka", it will be based on Gecko 1.9.2 and is expected to be ready by early-to-mid 2010. You can read all about it over here.
Firefox 4 will focus on better performance, Mozilla wants to make its browser feel faster and more responsive. Startup, time to open a new tab and responsiveness when interacting with the user interface should all improve. Namoroka will also feature enhanced personalization & customization, task based navigation, web application support and better integration with the operating system.