I thought the plan was for Firefox to use the same process-per-tab model as Chrome, but this post does seem to imply that it's just 2 - "we’re using project Electrolysis to split Firefox into a UI process and a content process."

They also go on to say that "After that, we’ll be working on support for multiple content processes.", so I assume the plan is to have one perocess-per-tab eventually. I'm surprised they aren't there already though, considering this has been in the works for a few years now.