Personally I think they should revive Positron, their electron competitor, with the launch of Servo. If it has the same benefits of Firefox vs Chrome, I think people would love apps that are on the slimmer side
I don't see the point, especially when there's Tauri, which is a slim wrapper around whatever the default web view is on the target platform. That's about as slim as you can get.
Mozilla making it would merely be a Firefox-based Electron competitor, which would probably end up being similar in terms of size. I think that should happen, but the goal shouldn't be to make apps slimmer, but to make a real Electron competitor (i.e. something that packages the browser with the executable).