Why am I prevented from downloading/installing the DownloadHelper add on?
When I try to download the Video DownloadHelper add on, nothing happens. A Firefox message advises that Mozilla has prevented me from downloading the add on.
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You should see a bar at the top with a button to allow that installation in such a case. If that doesn't work then download the .xpi file to your computer (right-click: Save Link as) and install the extension via File > Open File.