In mid-2015 Mozilla announced a big change to Firefox add-ons. This marked the start of the transition to WebExtensions - a new API (Application Programming Interface) for extension authors. The plan was that one day this way of writing extensions would be the only way, making the old extensions incompatible with Firefox. That day is right around the corner. On November 14th, Firefox Quantum will ship with only WebExtensions. Considering that for a long time extensions and extensibility in general were the big selling point of Firefox it might seem like an extreme measure from Mozilla to cut the branch they’re sitting on, especially in a time when their market share is being gobbled up by Chrome.
So, why would they do that? What was so fundamentally wrong about the old extension system that it had to be completely replaced? Well, usually when developers add extensibility to one of their apps they usually create an abstraction layer that sits on top of it where other developers have certain points they can plug into (hence the name plugins). This is basically how WebExtensions work - they do their thing and when needed they interact with Firefox in very specific points based on their needs and permissions. This allows the browser to keep them contained and away from its core resulting in increased security and performance.