These files are usually .xpi or .jar files.
Download the file to your local computer.
Click the menu button and choose Add-ons. The Add-ons Manager tab will open.
In the Add-ons Manager tab, select the Extensions panel.
To add the downloaded add-on to the list of available add-ons, drag and drop the file into the Add-ons window. The add-on is added to the list.
The installation process should begin.