When you download a file, Firefox will scan the file automatically once the file is completed downloading. This feature is good to protect your PC from virus attack. However, it may take longer time to download the file if the file size is huge.

If you have an up-to-date anti-virus installed, probably you can consider disabling this feature to get faster downloading speed. To disable file scanning, type about:config in address bar then press enter. Click “I will be careful, I promise!” button. On filter bar, type browser.download.manager.scanWhenDone, you will see the Boolean value set as true by default. To change the value to false, just double click on the entry.