I switched to Firefox for a week and wanted to kill myself. Basically, extensions still aren't forced to behave. As soon as you install a non-e10s extension, it disables e10s entirely, and then you're in the bad old days of single-process train wreck.
LastPass specifically seems like it has a bug where it locks the UI (beachball) while it populates login fields, and if you've got a lot of passwords (which we do), it'll take several seconds to unlock. Basically unusable.
I know these sound like LastPass problems, but the browser needs to sandbox these sketchy things better. I believe that getting from here to there is a really tough ecosystem problem, and while I appreciate Mozilla's philosophy on the open web, Chrome is simply better.
That's how I learned to spell the word "necklace"