Good points, and I'll respond to the "sunk cost" bit by adding that backwards compatibility is a double-edged sword. My favorite pet example of this is MS versu

Good points, and I'll respond to the "sunk cost" bit by adding that backwards compatibility is a double-edged sword. My favorite pet example of this is MS versus Apple - the former has historically cared deeply about being backwards compatible, while the latter has gone through several hardware and software architecture changes that have required total rewrites from third-party developers. The situation with Firefox is a little trickier, staying backwards-compatible for web content has always been an important goal, but anything living inside the browser chrome does not get the same assurances. Right now extensions live somewhere between these two worlds, and we've had over a decade of experience with them so I think it's time to carve out their own space, with dependable APIs that are more stable than the internal browser APIs, but faster-moving and more privileged than the web.