The actual uninstall step in this deployment package doesn't specify the version number to be uninstalled. It simply runs the uninstall tool included in Firefox with a parameter. I'm assuming the same tool is present in multiple Firefox releases, i.e. can be used to uninstall any Firefox installation on the machine.
It's the condition steps earlier on in the deployment package that check whether the Firefox installation is version 26.0. You could simply remove these steps (2 and 3) to uninstall all Firefox versions, and reinstall your preferred one. More information on creating and running deployment packages can be found in
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