Firefox 3.7 Design Mockups:
The design mockups of Firefox 3.7 look like they are making use of Aero Glass in Windows Vista and Windows 7. The RSS and bookmark icons have been moved out of the Firefox address bar and the two new entries Page and Tools have been added to the header. Page apparently contains functions and options that are directly related to the active page while Tools will be used for changing the UI of the web browser. Tabs are now rounded as well.
The design for Windows XP is basically the same as for Windows Vista sans the Aero Glass effects.
Firefox 4.0 Design Mockups:
Two different design mockups have been posted for Firefox 4. The main difference lies in the location of the tabbar. Firefox 4 version a comes with the tabbar in the usual place while version b moves the tabs to the top of the web browser similar to the tabs in Google Chrome.
Some of the most obvious changes include the absence of the status bar, the unifcation of the address bar with the search bar and a big new bookmarks button. Version b seems to have drawn several design inspirations from the Google browser. It does not come with a title bar anymore which could be a problem for long page titles. The bookmarks button is gone in this release and the page and tools buttons are placed in the tabbar next to each other.
The Mozilla team added a pro and con list to the second design mockup which is addressing all these concerns.
Verdict: Many of the changes proposed can already be done with add-ons which is probably a better way of handling changes. Add-ons allow users to remove the status or title bar, move the tabs on top or combine the address field with the search field. The biggest risk will be design changes that most Firefox users do not like or do not want to cope with. It is possible that the Firefox community will create add-ons that address these problems. It could also be that Firefox users decided to look at alternative browsers instead.