The goal must be a single controller ("UI") process and multiple content processes for each tab, otherwise they'll be missing out on the advantages of a multiprocess model, namely, stability and isolation.
They're first going to do two processes, probably for debugging without too many moving parts, then multiprocess. See https://wiki.mozilla.org/Electrolysis#Schedule_and_Status and https://wiki.mozilla.org/Electrolysis/Multiple_content_proce....