Servo is a research project, not a project to bring multiprocess support to Gecko. (Presumably you mean multiprocess support, not multithreading—Gecko is already highly multithreaded.)
In any case, Gecko is multiprocess already, and it's fully deployed on Firefox OS (and, as of recently, sandboxed). Firefox is not multiprocess because (generally) the front end has not yet been ported, and the project to do that is called Electrolysis.
In any case, Gecko is multiprocess already, and it's fully deployed on Firefox OS (and, as of recently, sandboxed). Firefox is not multiprocess because (generally) the front end has not yet been ported, and the project to do that is called Electrolysis.