"Photosynth for Windows" could be thought of as three individual pieces:
1: The Photosynth desktop application
2: The original Direct3D Photosynth viewer plugin for IE and FF 2/3 on Windows
3: The Silverlight viewer which is currently the default viewer
1: The Windows desktop application automatically checks for updates each time it is launched. I believe that this is also the case for the Direct3D viewer web browser plugin.
2: The Silverlight viewer should load any updates to the Silverlight app as it downloads any other updated part of the website.
In short, Firefox is notifying you that the Direct3D Photosynth viewer (whose development was cut off in favor of the Silverlight viewer in late 2008/early 2009) needs to be updated because Firefox made a breaking change to how Firefox released Firefox 4. Unfortunately, this was after the D3D viewer's development had been abandoned, so so there will never be an update that makes it work with FF4+.