In case you didn't get around to xpi fiddling, I just modified a greamonkey xpi to include a copy of jquery. I don't know if it works but I'll host it at http://jquery.offput.ca/greasemonkey-0.6.4-fx.xpi for anyone willing to try.
Or you could roll your own greasemonkey xpi with jquery added in. then you could reference it through the chrome:// interface afforded to firefox. Same technique you just don't have to rely on the jquery server.