In Windows Firefox automatically uses the local virus scanner, if installed, to scan any file it downloads. While this may provide extra safety, the real-time scanning functions of modern antivirus programs render this additional scanning unnecessary and causes delays, false positives and even freezes when downloading files – especially large ones.

To disable Firefox's automatic virus scanning perform the following steps:

1. In the address bar type about:config
2. In the filter bar (CTRL+F) type browser.download.manager.scanWhenDone
3. Double click this entry to toggle it from the default true to false
4. Close all open Firefox windows and restart to apply the change