"Accept cookies from sites" remains unchecked in Forefox

In Firefox, something strange started to happen today. Trying to access iGoogle and Yahoo! Mail, they kept telling me that my browser does not accept cookies. How is that possible? I checked my Privacy settings, and there it was, my "Accept cookies from sites" option was not checked. How did that happen? I don't remember unchecking it. No problem, I checked it and started reading my Technology news from iGoogle and checking my email. Some time afterward, guess what? Same problem! Now, I am concerned. I'm thinking my browser is being exploited somehow. But something does not make sense here. By not accepting cookies, I'm actually restricting access, not giving access, I'm increasing my privacy, not decreasing it. OK, so I'm not being exploited, I'm not revealing any private information through my browser, but at the same time, the cookies option somehow gets unchecked. Without keeping you into suspense for much too long, the problem was with one of my browser extensions, Stealther, namely it was being used. To be honest, I do not remember starting it (checking it under Tools -> Stealther), but that my friends, is another story.