Since July, we have known about Mozilla's plans to overhaul the Firefox user interface. As reported on Download Squad, a Mozilla dev has shared some newly updated mockups and information regarding the new look. On his personal blog, Chromatic Pixel, Stephen Horlander shares oodles of information, ranging from the design change itself, to the reasons behind the changes.
The App Button seems to be a major focus in the new UI. Many variations are being explored (see image at bottom of post), though all of them aim for the same things, simplicity, and space saving. The new button is set to replace the menu toolbar that Firefox users are accustomed to. He also links to a previous sketch of his, showcasing what the unified menu might look like. In the image below, you can see just how much space the new layout saves when compared to Firefox 3.5.
Much of the inspiration for the new App Button comes from Microsoft. Horlander lists the advantages of it as follows. It's less complex, takes up less space (leaving a less cluttered look), provides a unified location for menu items, and gives it a similar look and feel as the rest of Windows 7 and Office 2007/10. He also notes that the UI changes are scheduled for Firefox 4.0 only, and will not make it in time for 3.7, as originally planned.
Thanks to +Mephistopheles, in our forums, for the heads up.