Steps:
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.