How to fix "This add-on could not be downloaded because of connection failure" issue

[ashwin]

How to fix "This add-on could not be downloaded because of connection failure" issue in Mozilla Firefox.

1. Go to the add-on's page. In my case, it was Enhanced Steam. It wouldn't download for me, despite the fact it did download, but the installation failed. My guess is that Firefox 57 blocks something.

http://www.enhancedsteam.com/download/

2. Right-click on the add-on file (XPI).

3. Save it to your PC using your web browser or download manager.

4. Open the following page in Firefox.

about:addons

5. Drag the add-on XPI file to the page, and it will be installed.