Mozilla Firefox will not install Adobe Reader from web sites until you add them to the Allowed List. Please follow the steps below.

Click on the “Edit Options…” button in the bar to continue with download

Click on the “Allow” button to add the site to the list of allowed sites
Click on the “Close” button to continue with download

Click on the refresh button on your browser.

Click on the "Install Now" button to continue with download.

Click on the dialog close button.