Normally, whenever a file is downloaded by your browser Firefox 3.0, it launches a virus scan. This unfortunately can last very long. Especially since this feature tends to bugger with no real apparent reason. The larger the file is large, the longer the analysis is.
To disable this feature that may overlap with your anti-virus, anti-spyware, anti .... In the address bar of your browser type the following command:
Then click on I'll be careful, I promise!
In the filter box, type the following option: browser.download.manager.scanWhenDone
Double click on the option, the value fields must be False.