Somehow "Trojan Agent" Keeps Getting In Through Firefox...

Hello,

I'm having a recurring problem where, despite the use of a firewall and not visiting with my computer any sites that I would normally associate with viruses or other security risks, somehow every time I perform a security scan of my home directory with ClamTK, I come back with between 4 to 7 risks-found, one or two of which is always a "Trojan Agent" file. ClamTK says the source is Firefox. I tried ditching the outdated/no longer supported Adobe Flash Plugin, thinking it might be the "weak-link," but the problem persists.

What can be done about this?

Thank you for reading, and in advance for any information you might be able to provide.