Google started to test the unified menu at the pure style of Opera. Firefox 4 will include a single menu button. The unified menu takes up less space, it's less complex and it reduces clutter..
The new menu includes most of the options that were available in the page and tools menus. If you use a recent Chromium build or Google Chrome dev channel, you can enable this feature by adding a command-line flag to the desktop shortcut: --new-wrench-menu.
This new way to display the menus is less complex, save a lot space and looks cool, at least that's what I think :D
The main goal of this kind of menu is to show all the functionality of the browser (some of them wasn't available on the GUI) and save some space.
Source: Google System Blog