[ChristyD]

Rorschach112 did such a great job for another machine earlier, I'll try again with this machine's issues...!

Anytime I try to redirect to any site after my homepage within Firefox, it comes up with the following page:

Insecure Internet activity. Threat of virus attack
Due to insecure Internet browsing your PC can easily get infected with viruses, worms and trojans without your knowledge, and that can lead to system slowdown, freezes and crashes.
Also insecure Internet activity can result in revealing your personal information.
To get full advanced real-time protection for PC and Internet activity, register Antivirus 2008.
We recommend you to protect your PC now and continue safe Internet browsing.
Click here to get full advanced real-time protection and continue browsing.
Continue to this website unprotected (not recommended).

Internet Explorer works fine.

Full Scan with MalwareBytes didn't find any infections. I don't dare go further without an expert!

Thank you for your time!!