You don't have the cursor sitting at the top of the Firefox window, do you?

Firefox will un-hide the Toolbars in Fullscreen mode when the cursor is moved to the top of the window.

My cursor isn't at the top of the screen

See http://kb.mozillazine.org/browser.fullscreen.autohide

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.