Why would you think Mozilla Firefox has a firewall its just a browser
How to stop pop-ups and remove fake software, spyware, trojans and viruses:
Download, install, update and scan your computer with both Malwarebytes and SuperantiSpyware, delete anything they find, if this fails to completely remove the virus/spyware download then update Avast and run a boot scan (click schedule boot-time scan and restart your PC), all the software below is free and safe to use.
Malwarebytes: http://www.malwarebytes.org/mbam.php
SuperantiSpyware: http://www.superantispyware.com/
Avast Anti-Virus: http://www.avast.com/eng/avast_4_home.html
To protect your PC in the future you'll need to install anti-virus software and a firewall, id recommend using Avast and Comodo Pro firewall both are very effective and completely free, you should also switch to Firefox 3 if you are browsing with IE as its much safer and a lot easier to use. (links on my profile)