Are they? I've built a few Firefox extension and I cannot recall any part of the API that is Gecko-specific. While parts of the API aren't entirely browser-agnostic, they definitely seem to be engine-agnostic.
True, but I don't see how that answers the original question of "why does Firefox not support extensions on iOS?". Clearly extensions can run on WebKit. I assume it has much more to do with Apple restrictions rather than with the fact addons are built for Gecko.
Its not as simple as extensions running on webkit. Webkit is just a component. Extensions are basically apps. Apple goes through a lot of trouble to make it so you cant build an app platform inside your own app. Not saying impossible. But non trivial. Its not as simple as “webkit runs extensions”
Are they? I've built a few Firefox extension and I cannot recall any part of the API that is Gecko-specific. While parts of the API aren't entirely browser-agnostic, they definitely seem to be engine-agnostic.