It makes no sense to me, but having multiple different browser instances open (like, literally five different browser instances right now) seems to use memory more efficiently than if I had opened everything in multiple tabs in a single browser.
I started using site-specific browsers again, which I had started playing around with literally nine years ago.
Alas, Mozilla Prism is long-dead, but Fluid.app is still around.
But what I prefer the most right now is Epichrome.
So far I have a process running Facebook, another running Feedly, yet another running Google Hangouts, and the last one running GMail as well as a normal Chromium process. I will sometimes run out of memory in Chromium, necessitating a restart, but so far OS X 10.11 El Capitan seems to be doing a good job keeping memory usage under 7 GB (I currently have 8 GB installed on the machine I’m typing this on.)