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.
This blog post requires some very clear translating before all of grab our pitchforks. Users will still be able to install extensions from outside Mozilla’s own add-on website, and developers will still be able to distribute them separately. The functionality Mozilla is removing from Firefox is the ability for application installers – such as Skype – to dump an extension in a folder and then have that extension be installed in every Firefox profile on the machine.
This still breaks any Linux distro shipping GNOME from being able to install GNOME Shell extensions through the website since they need a Firefox extension to do it.