Multiple tab processes (aka "e10s-multi") should be shipping in Firefox 54 (now in the Firefox Nightly channel). The initial rollout of multi-process Firefox ("e10s") had one process for the browser UI and one process for all tabs (and a separate process for sandboxing Flash), so one bad tab could lock up all your tabs but your browser UI would still be responsive so you could close the bad tab. e10s-multi will increase the number of shared tab processes to two or more, balancing tab isolation vs memory overhead.

In the meantime, you can test more tab processes by increasing the "dom.ipc.processCount" pref in about:config. I set my process count limit to 999. :)