Sounds like your Firefox version is out of date. Multi process Firefox has landed quite a while ago, with different memory and performance characteristics.
Fortunately they didn't : they use one process for the browser UI, and a few (like two or four) processes for tabs which are dispatched between those process. It's way better than Chrome in term of memory consumption when you have lots of tabs opened.
I think, the number of processes (by default) is set to how many cores you have on your CPU. That way you can achieve almost the maximum of parallelism with a minimum of RAM use.
