On OS X 10.7.X, the new Lion-style fullscreen mode (implemented by bug 639705) ignores the browser.fullscreen.autohide setting, and doesn't autohide toolbars. But on OS X 10.8 it does. Thus it mixes elements of the "old" presentation mode (which supported autohiding) with elements of the "new" Lion-style fullscreen mode. Some might say this isn't a bug but a feature. See bug 740148.
Oops, this is fixed in current trunk code by the patch for bug 751955.
Commenting to express support for this behavior, which as mentioned is the desired mode for many. See bug 740148, bug 639705, and https://support.mozilla.org/en-US/questions/932384