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.
During the release cycle for Firefox version 73, which goes into pre-release channels on December 3, 2019 and into release on February 11, 2020, Firefox will continue to read sideloaded files, but they will be copied over to the user’s individual profile and installed as regular add-ons. Sideloading will stop being supported in Firefox version 74, which will be released on March 10, 2020. The transitional stage in Firefox 73 will ensure that no installed add-ons will be lost, and end users will gain the ability to remove them if they chose to.
If you self-distribute your extension via sideloading, please update your install flows and direct your users to download your extension through a web property that you own, or through addons.mozilla.org (AMO). Please note that all extensions must meet the requirements outlined in our Add-on Policies and Developer Agreement. If you choose to continue self-distributing your extension, make sure that new versions use an update URL to keep users up-to-date. Instructions for distributing an extension can be found in our Extension Workshop document repository.