When you are on another site other than mozilla.org, like many of the home web sites for extensions, and you click "Install" for an extension, a bar may pop up just below your toolbar. It will probably say To protect your computer, Firefox prevented this site (site name.com) from installing software on your computer., with a button named Edit Options ... in the far right corner of the same bar. Click the "Edit Options ..." button and a small box named "Allowed Sites" will pop up. It will have the name of the web site highlighted under "Address of web site:", with a button named Allow below and to the right side. Click the "Allow" button, then the "OK" button at the bottom. This adds that web site to your list of "Allowed Sites" from which Firefox will download software. Then, you can click "Install" for any of the extensions listed on that particular site.