When you're working in the Inspector's console (and especially after bug 14354 and bug 14355 are implemented), it would be nice to have a "clear" keyword that will clear the contents of the console. AFAIK, Firebug doesn't have anything like this to work off of.
Hooking up Ctrl-L/Cmd-K on the Mac to clear the console would be nice too.
It isn't a keyword but function clear() was implemented in Bug 19873