On a Mac, as far as I know, Safari uses the smallest amount of memory. (It's the only browser I've used on OS X for a while.)

I'm surprised to see Firefox taking up 3.5GB of RAM. On my Windows 7 PC, it takes about 1GB after a day's browsing (without Flash). I usually have 80-100 tabs but only 15-20 are actually open. It's fine unless Flash goes ape and eats 2GB of RAM for no reason.

I also use Vivaldi and Opera on Windows 7. Both are based on Chromium and both consume more RAM than Firefox. Vivaldi does "lazy loading" so it only loads tabs when you click on them, whereas Opera still tries to load everything.)