For debugging development purposes I am keeping a version now for older dev plugins that never got a proper replacement, and aren't compatible with the newer API and newer will be (because the API is so much less powerful, good for noobs / bad for power user).
And I can't wait until Servo gets more stable and feature complete. I am thinking about a Vivaldi browser like HTML5 based UI based on Servo web engine, instead of blink - that would be the ideal next-gen Firefox reboot! (Firefox/Firebird/forgotthename was initially a leaner fork of the "bloated" MozillaSuite, in the meantime Firefox got so much more bloated than the still maintained MozillaSuite (now called SeaMonkey, based on recent gecko engine)).