For the uninitiate, after you install FF3:
Right-click on any shortcut to Firefox's main .exe file (or just right-click on the .exe itself),
Select the [Compatibility] tab,
Choose Windows 7 or previous,
If you've already installed the Photosynth app for Windows, you should be all set.
P.S. If anyone knows how to set the compatibility mode for IE 10 Desktop, feel free to link me to some instructions.