This answer is no longer correct due to native integration of this feature into Firefox. Look to other answers for better options.
By default there's no way to do this in native Firefox, but there is an extension called Toolbar Autohide that should do exactly that! It allows you to use your mouse to make it appear again by hovering at the top of the screen where the toolbar would normally be.
Simply install the extension at that link and restart Firefox when it prompts you to do so. After Firefox restarts, right-click the toolbar and select
Maximized Autohide; you should then be ready to go!
I would recommend changing an option for the add-on in the add-on manager (
command + shift + a), though: under
Transition Type to
Slide-in to make it a little prettier.
Lastly, keep in mind that hovering your mouse at the top of the screen can make the OS X bar appear as well, which can be mildly annoying (which would happen for almost any extension that does this type of thing). To get around this, make good use of key commands!
command + l brings up the address bar so you can immediately start typing in a new URL
command + k brings up the Firefox menu so you can select your print options, preferences, etc.
command + t creates a new tab and focuses on it
control + tab and
control + shift + tab allows you to cycle through tabs just like
command + tab and
command + shift + tab allows you to cycle through windows in OS X
Hope this helps!