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