Depending on what version you can press F11 to hide all the bars at the top. Press F11 again to get them back.
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Probably your are trying to capture screen shots in a full screen mode. You can do it by using userchrome.css file, for your help, please navigate here, http://kb.mozillazine.org/index.php?title=UserChrome.css&printable=yes
But if you feel this is hard to do, better do the same by this add on, which is here, https://addons.mozilla.org/en-us/firefox/addon/hide-navigation-bar/?src=search