Flash already runs in a different process, even without e10s enabled.
Dunno why your Firefox is crashing every time it encounters Flash or similar media, but I can assure you that's not a common experience. Maybe try enabling click-to-play?
I filed a bug report for that, too, and it discovered multiple gaping holes in the sandbox, which, luckily, only allowed null pointer dereferencing (so no RCE problems, but still DoS)
Dunno why your Firefox is crashing every time it encounters Flash or similar media, but I can assure you that's not a common experience. Maybe try enabling click-to-play?