Here's the link:
https://addons.mozilla.org/en-US/firefox/addon/7023
and here's a direct link to the program 'creator':
TyCamTech.com
I love it. When I've got someone on ignore, all their posts are invisible, even if someone else quotes them, and it's easy to both put people on ignore and take them off of it. Also, it eliminates the "post unavailable because XXXX is on ignore" stuff that you see when you just use the 'ignore' function provided by the site itself.
It also allows you to ignore entire threads and to use the 'keyword' feature to ignore certain words that the site otherwise allows. If you hated the word "octopus", for example, you could censor it from your posts while the rest of the site could still read it.