A plug-in called PlaySushi Textlinks has been installed and activated without my permission. Can't uninstall it. What do I do?

I have Windows XP and Firefox 3.6.13. 2 days ago I noticed that random-seeming words in web articles and blog posts were underlined and hot-linked. Today I checked Add-Ons and noticed something called Play Sushi Text Links. I never installed Play Sushi Text Links. I've never heard of it before. If I click Uninstall, I'm prompted by Firefox to restart, but when the system restarts it shows that 1 add-0n has been activated. Play Sushi Text Links is still active. If I click Disable, Play Sushi Text Links is disabled for that browser session but is re-enabled when I start a new browser session. How can I get rid of this Play Sushi Text Links? And how was it installed on my computer in the first place? Thank you