One could say that it doesn't matter how many different privilege separation levels Chrome has if it so readily exfiltrates your data to Google's servers.

True. But security considerations add an important dimension to the conversation which is often missed.

Firefox has more than two process classes. I'm probably missing some still, but it separates at least into main process, content processes (tabs), NPAPI plugin process and extension processes (at the time of his writing, I believe this was one big extension process still).

There's also a process for Asynchronous Panning and Zooming (APZ), but that probably doesn't help much with security.

Chrome makes sure that no one get user data but Google.