Even though the Mozilla developers initially said they wouldn't do it, here it is: Firefox 4.0 (nightly build) finally gets an unified menu button. But not the way it's integrated in Windows (in the titlebar), but next to the tabs.
Here is how Firefox 4.0 for Windows looks like:
And here is the new Firefox unified menu for Linux (from the latest Firefox 4.0 nightly):
To get the new unified menu, you need the latest Firefox 4.0 nightly build which you can either install from the Mozilla Daily PPA (I do not recommend it) or by simply downloading the latest Firefox 4.0 nightly from HERE (to use: extract it, then double click the "firefox" file - important: not firefox-bin!).
Once you have the latest Firefox 4.0 up and running, go to View > Toolbars and uncheck "Menu bar" - then you should get the new unified menu.