While Firefox 4 is still in beta and even though it seems it will be released in March, it might actually be further delayed, the first Firefox 5 mockups are already starting to emerge.
It seems that Mozilla will adopt something called "site-specific" browsers (this is basically the old Prism with a new look) or "desktop apps" starting with Firefox 5, which will have an integrated site menu. Here are some mockups to understand how that will work:
Further more, Firefox 5 which is scheduled to be released by the end of the first half of this year (hopefully we won't start seeing delays like for Firefox 4 again) will get an updated search bar that will sport a 240 pixels width and instead of the favicon it will use the search provider main color (green for Google, purple for Yahoo, yellow for Bing and so on):
These are just the initial mockups so things might change. Also, we'll probably see a lot more changes in Firefox 5 as well as 6 and 7 - because Mozilla is planning to release all of these next year.
For more mockups, see: