Firefox 3 makes a call to your anti virus software to scan your down loaded file. This typically results in a double scan or an unnecessary scan when you trust the download source.
There is no intuitive way to disable this feature but thanks to gHacks I was able to find a configuration setting for this feature.
You can navigate to about:config in Firefox and double click the browser.download.manager.scanWhenDone to set its value to false. No more time wasted scanning the file.