I think you misunderstand what a browser engine is, then—read the Wikipedia article I linked and tell me how the “stand-alone product” that Servo is going to be

I think you misunderstand what a browser engine is, then—read the Wikipedia article I linked and tell me how the “stand-alone product” that Servo is going to be is not Firefox itself. Massive parts of Gecko have already been simply deleted from the codebase and replaced with components from Servo. “Firefox” is not a browser engine, it is a browser UI. Gecko and Servo are browser engines.