This style decreases the vertical size of the navigation bar and menu bar, which increases the vertical size of the browsing area by 1/8" = 3 mm (for example, for my setup). Credit to np & XerBlade.
You can edit in the code the number of pixels (e.g. -5px) to find what looks good for your setup. The current numbers look good for Firefox 1.5 for Windows using small icons and the theme "Firefox (default) 2.0". Note that in the default theme there is a very thin black horizontal border between the menu bar and the navigation bar that will probably be lost when you apply this style.
(I just noticed an extensive style that reduces the size of most of the GUI elements of Firefox, Slim down Firefox GUI. Its comments indicate it works really great for some setups.)
Works like it says.
Very Nice!