Firefox has finally been outfitted with simultaneous multiple content processes, a UI process, and a GPU acceleration process— eight years after the project, codenamed Electrolysis (E10S), began. Mozilla is calling Firefox 54 "the best Firefox ever," and they're probably not wrong (though Firefox 3.5 was pretty good, in my opinion).
In theory, moving to multiple content processes will improve stability and performance (one bad tab won't slow down the rest of your computer). Electrolysis is also a prerequisite for full security sandboxing in Firefox, which is currently only available for a few media-decoding plug-ins such as Flash.
The trade-off with multiple processes, though, is memory overhead, because each process contains an instance of the browser's rendering engine. Mozilla says they've worked hard to avoid increased memory consumption, but as a result you only get four content processes by default. Apparently that's the sweet spot between using too much RAM while still taking full advantage of multi-core CPUs. If you want to be more (or less) aggressive, you can visit
about:config and tweak
dom.ipc.processCount. By default Google Chrome starts a new process for every tab, which is one of the reasons it's such a memory hog.