The rendering engine is not the part that makes it hard to do multiprocess Firefox. Gecko has had the ability to do multiprocess stuff for years; all the stuff

The rendering engine is not the part that makes it hard to do multiprocess Firefox. Gecko has had the ability to do multiprocess stuff for years; all the stuff on the b2g process was multiprocess Gecko. The hard part that took this long has been updating the browser UI to not directly poke at the web content (which is now in a different process) and not breaking all the extensions which like to do that sort of thing too badly.