Back in February I posted about a proof of concept script that extracts the name of the currently logged in LJ user and maps it to their user ID, as found in the per domain cookies LJ produces. That script only worked in greasemonkey 0.3.
I've just updated it to work under the 0.5 security model and changed the regexp to work on LJ's current pages. If you're interested in writing scripts that need to know what the current user name is, give it a go, and let me know what sort of Firefox/greasemonkey versions it will run under.
Get it from userscripts.org.