I found it extremely frustrating when saving a batch of images to disk that Firefox would insist on virus-scanning every one of them, consequently causing the browser to lag.

Fortunately, there is a cure. Point your browser to about:config, and look for this entry:

browser.download.manager.scanWhenDone

Set it to “false”.

Highly recommended if you regularly save a group of small files to disk in a short amount of time.