If you make that particular setting "False", it will prevent your antivirus from scanning downloaded files in Firefox.. NOT a good thing unless you really feel like you want to manually RIGHT click on every downloaded file and scan it with your antivirus program. Generally, you DON'T want to wait for the downloaded file to be opened but instead want it scanned when it's "saved" to the computer.. The value also indicates whether downloaded files trigger a security warning upon launch. I recommend leaving it enabled/True.. but it's your choice.
Read the link below for the basic information about "browser.download.manager.scanWhenDone":
Hope this helps.
There is a Firefox option to skip scanning of downloaded files. Disabling this scan nominally speeds up the browser a bit.
I'm using AVG 8 on Win XP Pro, browsing with FF 3.
I presume that AVG scans all files when they are opened, so scanning files on download does seem redundant. Or am I missing something?
The Firefox key in question is browser.download.manager.scanWhenDone and the default value is True. I have not changed it, but I do wonder whether this extra scan adds anything from a security perspective.