A browser plug in automatically opens an embedded file or a file specified by an URL. For example, Acrobat Reader has a plug-in for Firefox so that Firefox automatically runs Acrobat Reader when an HTML document embeds a PDF file, or the URL specifies a PDF file.
This is bad from the perspective of virus protection. PDF files can carry virus payload. Although clamfs can detect a virus embedded in a PDF file, it does require an end user to set up a clamfs mount point as the browser cache and default download area.
If you turn off the plug-in, then you will need to manually download a file and open it separately. This gives you a littie more time to think about whether the file should be opened. clamav does integrate with many file managers (such as Nautilus) to make file scanning easy and painless.