Perhaps the most secure way to keep your Firo wallet is on the Linux Qubes OS, on the Lenovo NitroPad X230 or T430. This is not feasible for most users because Qubes has a high learning curve.
Second would probably be on some Linux distribution like Kodachi, or even Apline Linux. Alpine Linux would also reequire installing extra software like Python to get it to work. Even Ubuntu would be better than Windows and FIRO can easily be installed and work right out of the box.
Perhaps the most secure hardware wallet is the Ngrave, but that does not support FIRO.
Another way is to use a Chromebook which are actually pretty secure except you are limited to using android apps. That would mean using the FIRO android app which does not have the ability to setup and modify the wallet for use with Masternodes.
I would like to see the app eventually get to the point where you can setup masternodes with it. What is the likelihood of this happening? Using the FIRO wallet on a dedicated secure Android ROM such as Graphene OS or Copperhead OS would be ideal.