Firefox has a lof of configaration options using “about:” clause but we generally don’t use it, So here is a list of commands to pass your time 
about:
Just type the above clause and you will see general info about Firefox.
about:mozilla
Pretty scary, try it for yourselves.
about:blank
This as we all know it, shows a blank page.
about:config
This shows the main Firefox configuration.
about:cache
This shows the cache info & stats, and allows you to browse your cache files.
about:credits
This shows all the people who contributed towards Firefox development.
about:plugins
This shows info about all the plugins installed on Firefox.
about:buildconfig
This shows info about the particular Firefox build, that is if you wanna know.
about:robots
This is the funniest, check this out.
—Edit (thanks to blacknoise)—
about:sessionrestore
This is useful for manually restoring your sessions.
about:certerror
This is the window which is displayed to the user when there is a certificate mismatch occurs (if you see this screen, then thats probably a good idea to get out of that site asap).
about:privatebrowsing
This is useful to start private browsing mode.
resource:
Shows the directory structure of the Firefox installation.
This is all that I know, if you know any, I will be happy to add them to this list, please let me know in your comments below
yea man.. about:mozilla is really scary..
i knew most of them from an add on of a menu with these commands..
but the one i used mostly was about:config
and the first option i change was browser.search.openintab i set it to true
how about these
about:sessionrestore
about:certerror
about:privatebrowsing
(for Firefox 3.5)
and also there is resource: which is not an about command but it shows the directory listing of Firefox installation root
yea these are really new for firefox 3.5
i guess that about:sessionrestore is the one i may use most
about:license