I'm not completely sure, but I'd say that you'd have to have the latest version of FireFox downloaded and the .msi file in your packageshare.
I ran into the same thing when Chrome just updated again. I downloaded the latest .msi, went into the packageshare and deleted the old one and replaced it with the new one and updated the name of the Deployment Package to reflect the correct version. However, if you want to be able to look back and see when you did previous updates, you'll need to make a new Deployment Package and leave the old one alone. When you change the name, it also changes the name in your installer logs as well. Not that big of a deal for me, just FYI.