Firefox 4 isn't even out of beta yet, but that doesn't stop the UI team from thinking about Firefox 5.
ConcievablyTech uncovered mock-ups posted by the Mozilla UI team that show the possible UI for Firefox 5.
Here's what we have (click for larger images).
Search field changes:
Promotion of Firefox Sync:
The site-specific browser idea is interesting. Basically it consists of dedicated tabs that integrate into the UI and offer site-specific features.