When you download a file with Firefox, there is a delay for a few seconds before the button to save the file becomes enabled.
What's the motivation for this feature?
User Experience Stack Exchange is a question and answer site for user experience researchers and experts. It only takes a minute to sign up.Sign up to join this community
It is to prevent unintended downloads. If the user is attempting to interact with the webpage, for instance typing some prescribed text or double-clicking somewhere, without a delay, it would be possible for malicious websites to unexpectedly prompt for a download, and unintentionally be given approval by the user.
For more information, see this full explanation detailed by Jesse Ruderman who worked for Mozilla on the security team for quite some time, as well as a workaround to disable this security feature, if desired.