Mozilla is currently testing Electrolysis (or "e10s") [1] - their project to run web content in a separate process - on nightly, and I've been using it for the past few weeks. It's getting pretty reasonably stable, so hopefully we'll see it in a release version soon enough. According to [2], the tentative plan is for Electrolysis to be released with Firefox 36.
[1] https://wiki.mozilla.org/Electrolysis
[2] http://www.ghacks.net/2014/07/12/mozilla-plans-release-elect...