Can you give steps to reproduce this crash? It should definitely not happen. In fact, until https://bugzilla.mozilla.org/show_bug.cgi?id=558184 is implemented we rely on an installed Flash plugin to make the Flash detection used by most sites work.
0) Have a Macbook Air running OS X Yosemite
1) Install Firefox
2) Install Adobe Flash
3) Install Nightly
4) Install Shumway
5) Attempt to open http://www.albinoblacksheep.com/flash/shumway
6) Watch Firefox complain that the tab crashed
7) Try to reload the tab
8) GOTO 6
Disabling Flash (via the Addons screen in Firefox/Nightly) seems to fix the issue.