If you were short of change (like me) when you bought your monitor and purchased a small one (17´´),
this is a modest tip to gain a few cm. in firefox:
1) Install the "Compact Menu 2
" extension. That will replace the menubar with a simple button.
2) Create 1 or 2 folders to organize your bookmarks and move your "Bookmark menu items" to the Navigation Bar.
Here's a screenshot of what it might look like.