You're talking about a modular rendering engine, which is a different thing than what I'm talking about. I'm talking about stripping down the UI until it resembles XUL Runner, then adding the functionality back as extensions.

You're not wrong that it's important for the engine's code to be organized well for developers' benefit (and ideally for the engine as a whole to be self-contained -- it'd be great if Gecko were as easily-embeddable as Blink), but I'm not so sure that users need to be able to add or remove pieces of it in a way similar to what I'm talking about for UI features.

More concretely:

I think Firefox should ship by default with all the functionality it currently has, plus uBlock Origin and some other things. But I want it to be designed such that if you went into the extensions manager and disabled everything, things like tab support, bookmarks, history, and maybe even the address bar and back button would be gone. It would still be capable of fully rendering a web page, though.