Back in 2013 the increase from one process to two processes were 10MB.

https://billmccloskey.wordpress.com/2013/12/05/multiprocess-...

This is multiprocess, but not process-per-tab. There is still a single rendering process for all tabs, at least for now.

But the ultimate plan seems to be to have process-per-tab. The article states: "In future iterations, we expect every browser tab to run in its own process."

There's some interesting talk about that in mozilla.dev.platform: https://groups.google.com/forum/#!topic/mozilla.dev.platform...