I've been using Firestorm on Steam Deck a few weeks, and it runs great. I use it in Desktop mode so I can multitask and use browsers right and such, but it runs really good.
2 things I have noticed.
1. It will not always detect GPU memory on start up right, and can have issues in what capabilities it gives. You can go into Steam Deck bios and set the GPU to start with a min 4GB ram and then it works much better.
2. In Desktop mode, in Steam, you can program how all the controls work to customize exactly what you want for controls. It took me a while to get all types of keybindings to work the way I wanted. It works good.
Edit: 3. I will add I can get slightly higher FPS in Gaming mode vs Desktop mode... but then you cannot really multitask over to a web browser for links or anything because browsers run with too many problems in gaming mode.