Nowhere is it being proposed that you can no longer extend Firefox -- it's just that you will need to use a more user-friendly language than XUL to do it. It's called something else, and suddenly those that don't take the time to read the linked articles freak out and declare that the end of the world has come for Firefox. Does anyone not think that the Firefox team would have thought through the consequences and decided with sober mind that they're positive?
Another poster has already quoted the firebug response, but here goes again: "I don’t think these changes will have a big impact on Firebug. Firefox will continue to support extensions while the jetpack technology matures. We can adapt as we go along"
If the creators of an extension as complex and deeply embedded in Firefox as Firebug think it's going to be ok, then maybe that's a sign that we should all just take a big breath and chill out.