As an update to my previous post on trouble with Flash — Adobe has released an updated version now, 11.3.300.262, which is supposed to fix the crashes seen in Firefox with protected mode enabled, at least. A quick test with a Youtube video also didn't show any choppiness and/or 10-second stalls, but I'm not holding my breath there quite yet.
In any case, I wanted to give everyone a heads-up. Don't forget to reenable protected mode (if it turns out not to work for you, you can always disable it again)!
EDIT: turns out protected mode still causes problems, both lag and crashes, although the latter are less common now. Time to disable it again!