Last week we took a look at initial design mockups for Firefox 3.7, and now the busy designers at Mozilla are back with some theme mockups for the even-further-into-the-future Firefox 4.0.

(Click any of the images for a closer look.)

As you can see in the screenshot above, the Firefox folks are toying with the idea of a Google Chrome-like tabs-on-top interface.

They're also looking at consolidating the Stop, Refresh, and Go buttons to save a little extra space in the toolbar—similar to a user chrome tweak we’ve been using for quite a while.

Keep in mind that all of the changes you see here are not final—they're just exploring different ideas for upcoming releases of Firefox. Similarly, it'll be quite some time before either Firefox 4.0 or Firefox 3.7 hits servers, so if you like the basic look but want to get it now, here's how to make Firefox 3.5 look like Firefox 3.7.