Here’s how to stop Fire­fox from au­to­mat­i­cal­ly virus scan­ning all down­loads after they’ve fin­ished - some­thing that’s been an­noy­ing me since I’ve up­grad­ed to Fire­fox 3 be­cause of the ex­tra­ne­ous disk ac­cess­es it cre­ates. Open up the about:config (open a new tab, type about:con­fig in the lo­ca­tion bar), and find the browser.down­load.man­ager.scan­When­Done val­ue. Type in browser in the fil­ter bar for quick search­ing. Set that val­ue to false by dou­ble-click­ing. And that’s it! What, were you ex­pect­ing some­thing more?

(orig­i­nal­ly read here)