I do this instead of running antivirus in the background constantly, but that point of view is apparently controversial. Use the Download Scan Firefox extension and configure as follows:
Exclude these types: jpg, jpeg, gif, png, htm, css, asx
Path to clamwin.exe
Parameters: --mode=scanner --path=%1 --close
My opinion is that downloaded files are the main source of virus trouble, so scanning upon download is good for safety; where constant background scanning is overkill and a drain on system performance.
