After using Firefox for several days without restarting, my memory usage is extremely high. I have around 40-50 tabs open. Just now my memory usage was over 4gb and Firefox was extremely slow. After restarting and restoring all my tabs memory usage was only 700mb. I went and visited each tab individually to make sure they were all loaded and it climbed to about 2gb. This is a lot but not too unexpected for this many tabs open. I'm not sure how this doubles while keeping the same tabs open, though.