highlightwords.user.js
I've also wanted to have a Mozilla keyword that opens multiple websites and I finally set it up. It's easy to do, just make a new bookmark with a keyword and have the URL be something like:
javascript:window.open('http://wtwf.com/scripts/');window.open('http://www.fark.com/');window.open('http://slashdot.org/');
You can make that as long as you like and it'll just work. You might need to be using Tab Mix Plus to make sure that unresized popups end up in a new tab rather than a new window, but you should be using that anyway.
In case you didn't know you can do this a lot easier from the command line (or a window manager menu or some such like this:
firefox http://wtwf.com/scripts/|http://www.fark.com/|http://slashdot.org/