Sideloading is a method of installing an extension in Firefox by adding an extension file to a special location using an executable application installer. This installs the extension in all Firefox instances on a computer.
Sideloaded extensions frequently cause issues for users since they did not explicitly choose to install them and are unable to remove them from the Add-ons Manager. This mechanism has also been employed in the past to install malware into Firefox. To give users more control over their extensions, support for sideloaded extensions will be discontinued.