If you look down the list of versions, you'll see that 0.9.20 adds site whitelist/blacklist.
Hope that helps!
Lobster
PS - If it's for some type of parental control, I'm not sure if that blacklist is password protected or not, so it may be easy for a kid to simply turn off the Firefox extension as they learn more about computers.
Internet Explorer has the embedded content filter for that as well, or you may want to consider a 3rd party parental control program.