Yep, Webkit dates to 1998, and Firefox (2002) was such a radical departure from Mozilla suite it's more than a little ridiculous in 2016 to handwave Chrome (2008)'s advantages as "greenfield." Microsoft just released a brand spanking new browser. Firefox is a great product but it's deficiencies are its deficiencies.
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.
