Firefox automatically scans the file after the download is complete which may cause exuberant amount of time in some low end computers. This function is vital to protect your computer from viruses, but you may find that it takes a lot more time in scanning if the file is huge. You can disable this feature if your antivirus is up to date and you don’t need Firefox to scan the file for you.
To disable this feature:
Type about:config in address bar and press Enter.
Click on “I will be careful, I promise!” button, now in the filter bar type,
You will see a Boolean value set as true by default just like its shown in the screenshot below.
To change the value to false just double click on the entry and it would change itself automatically.
And you are done. Enjoy !