Firefox 4.0 Early Screenshots

These images, all of Firefox in the Windows OS, reveal that the company is struggling to choose between two primary designs: tabs-on-bottom and tabs-on-top. Tabs on bottom is the current norm in Firefox, while the tabs-on-top looks almost exactly like the Google Chrome browser. Here is the tabs-on-bottom version (click for full-sized versions)

And here's the tabs-on-top design:

In addition, Mozilla summarizes the pros and cons of the two designs very succinctly:

Positives:
- Save Vertical Space
- Efficiency/Remove Visual Complexity – Right now the tabs have to be connected to something. So we are adding an extra visual element for them to connect to.
- Shorter Mouse Distance to Page Controls

Negatives:
- Breaks Consistency/Familiarity – Moving things confuses existing users.
- Title is MIA – With the space removed from the titlebar you only get the truncated version in the tab.
- Longer Mouse Distance to Tabs – Takes longer to mouse to a tab.
- Lost Space – Sandwiched in between the application icon and the window widgets you lose some space.

Combo Stop/Refresh/Go Button
Firefox 4.0 incorporates all of the design changes featured in Firefox 3.7 and includes a few new features. The big one is a Combo Stop/Refresh/Go Button.

Instead of separate buttons for each of these functions, they're all integrated into one button on the right hand side of the address bar. It also changes colors based on your actions. So if you're typing, it will be a green "Go" button, but if the page is currently loading, a red "Stop" button will take its place. Not a bad way to save space, if we do say so ourselves. Here's a screenshot of the new feature in action: (click for full-sized version)

This is all proposed, and feedback from the Firefox community shapes the browser into its final form. Read More

Checkout the Mozilla Wiki

[SOURCE: MASHABLE]