Just wanted to mention that the disappearing Firefox icons on the taskbar is still an issue for some users when upgrading. I deployed Firefox 10.0.3 ESR over 10.0.2 ESR the other day and, while my test station didn't see the disappearing icon, it happened to a couple of my users.
Upgrading Firefox is fairly seamless with the MSI, but the disappearing icons really take away from the experience.