Stylish script for Firefox to force toolbars to appear in fullscreen mode.

firefox-stylish-force-toolbar-in-fullscreen.css

@-moz-document url(chrome://browser/content/browser.xul) {
    /* Force toolbars to appear in fullscreen mode. */
    #main-window[inFullscreen][inDOMFullscreen] #navigator-toolbox,
    #main-window[inFullscreen][inDOMFullscreen] #fullscr-toggler,
    #main-window[inFullscreen][inDOMFullscreen] #sidebar-box,
    #main-window[inFullscreen][inDOMFullscreen] #sidebar-splitter,
    #main-window[inFullscreen]:not([OSXLionFullscreen]) toolbar:not([fullscreentoolbar=true]),
    #main-window[inFullscreen] #global-notificationbox,
    #main-window[inFullscreen] #high-priority-global-notificationbox {
        visibility: initial !important;
    }
}