Firefox was sometimes hanging on a single tab for me. I wanted to try Electrolysis, but I had three incompatible extensions. I disabled all three, restarted Firefox, and verified that multi-process was running in about:support (I saw 2 of 2 processes). Shortly thereafter, Firefox performance plummeted. I re-enabled one of the incompatible extensions and restarted Firefox. The performance returned to normal. From that I conclude that running Firefox multi-process can actually make performance much worse. I'm running on a MacBook Pro running Beta High Sierra, but I doubt that is related to the issue. My only question is how many other Firefox users have experienced performance dropping when using multi-process.