browser.tabs.remote.autostart should already be default-enabled, unless your extensions aren't compatible with it. It'll tell you "disabled by add-ons" in about:support, if that's the case.

And you can enable multiple content processes prior to Firefox 54 by setting dom.ipc.processCount to a number higher than 1. This is the maximum number of content processes that it'll use, as you open more tabs. So, if your number of tabs exceeds that value, it'll currently still distribute them among your content processes round-robin-style.

You get the best performance vs. RAM ratio with this value at 4 (or I believe rather just the number of cores your CPU has). This is also what Firefox 54 will ship with. You get the best security and best performance, disregarding RAM, by setting it to something really high, like 500, so that a new process starts up with each tab that you open (which allows each tab to be sandboxed individually).