Note: Starting in Firefox 57, which will be released in November 2017, only add-ons built with this new technology will work in Firefox. These are indicated by the "Compatible with Firefox 57+" label on
addons.mozilla.org (AMO). Add-ons built with the old technology are labeled "Legacy" in the Add-ons Manager (about:addons) tab.
If an add-on does not have the "Compatible with 57+" label or has the "Legacy" label, the developer may be in the process of transitioning to the new technology.
We are aware that certain add-ons will not be transitioning to the new standard; we will continue to assist developers to the fullest possible extent. As we get closer to November, we will suggest suitable replacements of add-ons not making the transition.
If you are a user of Firefox Nightly, please note that Firefox 57 reached the
Nightly channel on August 2, 2017. Legacy add-ons will be disabled, but will be enabled again if the developer releases a compatible update.
In the meantime, you can check if an add-on is
planned for migration (popular add-ons only),
look for alternatives or reach out to the developer to inquire if they will migrate their add-on. You may find developer contact information on the right side of the add-on listing on AMO.