What does the error message "the add-on could not be downloaded" mean in Mozilla Firefox?

About the error

Sometimes antivirus or other security software can prevent browser add-ons from being installed or automatically updated in Mozilla Firefox. This can also happen with the Amazon Handmade add-on offered by Vendio. If it happens you might see the error message "the add-on could not be downloaded because of a connection failure" (or a similar message).

How to fix the error

If you are having trouble installing the Amazon Handmade add-on in Firefox, or if it does not update automatically, please follow these steps to install/update the extension manually:

1. Open a different browser (such as Safari, Chrome, or Microsoft Edge).
2. Copy and paste the below link into your browser to download the Amazon Handmade add-on. It will be saved as a .xpi file in your Downloads folder.
3. The link is: http://scripts.vendio.com/hdm/amazon_handmade.xpi
4. Open Mozilla Firefox and drag the .xpi file into the Firefox window. Then click "Add".