When Firefox has finished downloading an application, image, video, etc.., It checks if there is no virus in the downloaded file. The problem is that the larger the file is, the longer it will take to audit. In addition, this feature seems to be ineffective.
If you want to disable this feature, follow this procedure!
Here, the feature is now disabled! But always scan a downloaded file with your antivirus.
Note that: on some versions of Firefox, this method can crash the browser after downloading. If this is the case then set browser.download.manager.scanWhenDone tp true