Hi Andy,
I think what may be happening is that:
1. You've supplied the samples in MP3 format
2. Firefox doesn't support MP3 files through the HTML5 API on Mac OS X
3. Firefox falls back to using Flash
4. Flash crashes
So one way to improve the situation for Firefox on Mac OS X users would be to supply ogg versions of all your samples because Firefox does support those through the HTML5 API. Extra work for you though obviously!
NB: Firefox on Windows uses the underlying OS support for MP3 to avoid directly supporting MP3 in the browser itself. They don't appear to do that on Mac OS X: https://developer.mozilla.org/…L/Supported_media_formats
Nick