From Firefox 57 onwards, Firefox browser won’t load any type of legacy add-ons. Latest Firefox 55 Nightly started showing the non-WebExtension add-ons with a legacy badge in the add-ons manager. By doing so Mozilla aims to warn users the add-ons they’re using are not WebExtensions. After clicking on Legacy, SUMo article will open in a new tab that offers detailed information on what is happening.
“This is the start of the plan for showing users what add-ons won’t make the cut for Firefox 57. It’s about adding a flag “legacy” into add-ons manager on each add-on that is not a WebExtension.”
FYI, starting from Firefox 53 onwards, AMO is no longer accepting new legacy add-ons. Mozilla believes the changes coming in Firefox platform such as Multi-Process Firefox (e10s), multi-e10s and sandboxing, could break many legacy extensions.
“Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the “Compatibility Milestones” document for more.”