It’s possible that, in your upgrade, your new template is using technically a different Firefox package than your old template. I would open the Qubes Manager Tool, then go to the AppVM you are having trouble with and right-click, and then go into Settings.
Under that menu there is a tab call Applications, and I have found sometimes I need to manually refresh the Application list using the button in there, and then make sure the firefox package you are using on the right side of the menu does not say “Application missing in Template”.