This is a totally fresh new over the internet, the first pictures of what Firefox 4.0 would be have leaked around. I’ll only explain what you can see in this pictures:

1) There’s no more: File, Edit, Blablabla… there’s now a more “Start” button like it’s on Microsoft Windows Vista & Windows Se7en & Office. 
2) Redesigned search bar and Icons. (Included the Glass or the Aero)
3) Generals graphic user interface aspects improved.
4) Also and obvious the performance has been improved but I’m also referring here about the graphic and the visual appearance of the program.

This are the proposes that Firefox team receive for the “Firefox Button”

And this is the Bookmark section added to the Firefox under the search bar

Source: Mashable.