When you try to install add-ons from third party sites from other than AMO, Firefox  shows a warning like this :

“Firefox Prevented the site(www.download.com) from asking you to install software ”

To solve this

  1. Open Firefox, click on menu > Options > Security
  2. Under General,  click on Exceptions for the “Warn me when sites try to install add-ons” option
  3. Then a windows opens, which lets you enter the web site address from which you want to install add-ons, type that site address, and click “Allow”and click close.

Firefox allowed sites Add-ons Instalaltion

4. Now try to install the add-on by visiting that site it will be installed now without any issues.

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