The unfortunate truth is that:
1] browsers do not prevent multiple windows from running simultaneously at maximum resources (which is extremely idiotic to say the least), so the resources are divided among all open tabs or windows, each one competing with all the others.
2) browsers have bugs that cause crashes
3) web-page or app developers that concentrate aggressively on ads, usually consume most resources.
4) an intensive Flash game or app might be demanding on resources -as is every other offline game or app on the PC, which is why the "exclusive mode" has been "invented".
Then the only solution is to provide that exclusivity artificially to Flash too:
Close ALL other windows, tabs and applications running in the background and try again. Chances are, it won't crash, at least on Firefox 30+ and Internet Explorer 11+.
I've found that the later might give a little more "room" in a demanding Flash app or game.