Firefox 5, the next version of Firefox will feature a tab multi-select and a new tab page. Add-ons will get a standardized toolbar support. Firefox will now have a file upload indicator. Instead of a home button, Firefox will have a "Home app tab."
Firefox will have an integrated PDF viewer and will eventually support formats like MP3. Firefox will also have taskbar web apps. Basically, tabs will have their own context menus while websites will act like "apps."
Firefox will feature an integrated identity manager with multiple sign-ons at the same time. Firefox will also allow you to "share" websites directly from your URL bar to social accounts like Facebook, Twitter, Digg, etc. You will not need to click the "Like" button anymore.
Firefox 5 is currently published as Firefox 4.2apre and the anticipated release date of the final is around June 29. Firefox 4, which recently launched, has been downloaded nearly 7 million times in the 24 hour period of its launch.
Here're a couple of screenshots: