The other day I accidentally unchecked all the toolbars including the menu toolbar in FireFox 3.6.
Here's one way I know of on how to restore the menu toolbar:
Click Windows Start > Run
Type in: firefox -safe-mode
Click OK
The follow dialog box will pop up.
Check "Reset toolbars and controls"
Then click "Make Changes and Restart" button.
The Firefox browser is launched with the menu toolbar enabled.