http://www.ghacks.net/2016/02/13/multi-process-firefox-will-use-up-to-20-more...
Firefox users should anticipate an increase in memory use when Firefox's multi-process architecture is enabled by Mozilla. The increase varies based on use, but it will be between 10% to 20% on average.
Overall we see a 10-20% increase in memory usage for the 1 content process case (which is what we plan on shipping initially). This seems like a fair tradeoff for potential security and performance benefits, but as we try to grow the number of content processes we’ll need to take another look at where that memory is being used.
