Not finding the Trojan...

[GreenDragonFly]

Hey, I've never posted on this site before and I'm not a computer whiz, but Spybot directed me here. I keep scanning my computer with Spybot (and Malwarebytes, and Webroot) and nothing seems to be finding what I suspect is a Trojan on my computer (even while in safe mode).

Anyway, what happens is that I click on some page (like Amazon, ebay, news sites... legit sites) and a new tab opens in my browser (Firefox). The tab is either for a random product I don't want, or some congratulations that I've been "selected" to participate in a survey (usually claiming to be related to Amazon or ebay, etc.).

I've used Firefox to block these pages (because I don't trust their content to be safe). But of course, then I just get a blank tab popping up, and a box from Firefox telling me that it just blocked a site.

Anyway, like I said, I don't know much about computers, but again, this seems like a Trojan that none of my software is finding. Any way to fix this (a walkthrough anywhere)?

Thanks!