[Osiris]

There is no obvious way in Firefox to reduce the size of the menu toolbar. I always thought that it was a big waste of space to have this bar enabled all the time. I’m personally using the Tiny Menu Firefox extension which displays just a Menu item next to the usual buttons such as forward, back and reload. I added a screenshot of how my setup looks like at the end of the article.
I did find another interesting Firefox extension called Personal Menu which reduces the menu bar to three buttons that can be placed in the other toolbars. Two buttons are for the bookmarks and the history function while the third is fully customizable. You can add any kind of menu from the menu bar for instance to this personal menu.
You should add at least the Addons menu before you disable the Menu Toolbar. In case something goes wrong you need to be able to disable the extension again. Take a look at the screenshots to see the differences:

• The normal menu bar
  
  
• Firefox Menu with Tiny Menu Extension
  
  
• Firefox Menu with Personal Menu Extension