This time I actually read the Manual (XUL, that is)! This version has some fixes resulting of that.
Thanks to interest and suggestions from users, the add-on now has a total of 8 locales: de, en, es, fr, ja, pt-BR, pt-PT, ru.
Besides the original menu items I was also thinking of adding buttons, which was a requested feature.
Now the add-on has preferences so you can choose what features to use and keep your Firefox clean.
For the toolbar, there is a "Restart" button, an "Exit" button, and a button which combines both.
For "File" menu, those two items (as in the previous version), but now you can turn that off.
For Firefox4 only, the same items as the "File" menu under the Firefox button menu (again, like the previous version, but now you can turn off).
Finally, and also for Firefox4, two buttons that sit in the titlebar next to the minimize/maximize/close buttons.
Possible bugs (comments welcome)
This has only been tested by me on Firefox 4.0b3 and b4, and Firefox 3.6.8. It is possible some functionality doesn't work on previous versions. I assume it works from Firefox 2.0 because it's when session restore was implemented.
I haven't done a thorough testing using languages other than English. When I tested, it worked.
The position of titlebar buttons is adjusted to the minimize/maximize/close buttons of Windows Aero interface. It is possible that they won't show up properly with other interfaces/OSs. I don't know how to determine the position of the window buttons created by the OS, so the only fix I can remember is to include an option to specify the position of the add-on buttons.