Have you ever tried to develop a old-school-style Firefox extension?

I made an extension in both browsers three years ago. It took me a week to make the Firefox extension and one day to make the Chrome extension. Chrome extensions are basically websites.

I also think Chrome's permissions based system is superior from a user-security standpoint.

They're not really dismantling it, so much as trying to push it towards standardization. Some (most?) addons will get left behind, but they'll still accept patches for anything that the actively-maintained ones need.