This week we saw bug 856140 Update document.register to adhere to the latest Custom Element spec fixed. So you can now do things such as declaring and instancing <fancy-button>s, which you couldn’t do with the previous implementation, because it would require your custom element’s name to start with an x-, following the old spec. Also, UI libraries such as Brick and other X-Tag based libraries won’t break because the callbacks are never called when web components are enabled. Those are all old bad nightmares from the past!
However the spec isn’t finalised yet–hence you can’t be done implementing something which hasn’t been written yet. And we need people to experiment with web components in Firefox and report bugs if they find them (you can file them under Product = Core, Component = DOM).
If you want to try this out, you can download a Nightly copy and follow the instructions in my Shadow DOM post to enable web components support (it’s disabled by default to prevent “breaking the world”, as Potch would put it).