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.
*** This bug has been marked as a duplicate of 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