TL;DR: Firefox used to have a great extension mechanism based on the XUL and XPCOM. This mechanism served us well for a long time. However, it came at an ever-growing cost in terms of maintenance for both Firefox developers and add-on developers. On one side, this growing cost progressively killed any effort to make Firefox secure, fast or to try new things. On the other side, this growing cost pr
![Why Did Mozilla Remove XUL Add-ons?](https://cdn-ak-scissors.b.st-hatena.com/image/square/65038b4c1baebed995f5c4d9225a42d2cacc1a1d/height=288;version=1;width=512/https%3A%2F%2Fyoric.github.io%2Fimg%2Fdismantling_scaffolding.png)