You actually can; I'm doing so right now. However, you need to install the full Shumway extension from http://www.areweflashyet.com/shumway/ (the stripped-down version shipping in Nightly is limited to Amazon only).

If you use Shumway, though, don't forget to disable Adobe's Flash (I've found that having it enabled and Shumway installed results in rather brutal crashes).

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.

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.