I help develop the Photosynth viewer, it would be really helpful to track this down if you could provide a little bit more information about your environment, namely:
- Operating System (XP,Vista, OS X etc)
- Browser (IE7/8, FF3 etc)
- The amount of memory the browser is using when you see the crash
- How long do you have to be viewing a synth for this to happen?
- Is there any specific action that you are doing which causes the crash to happen, or it appears to happen randomly?