Tl dr: download the mouse toggle app so you can click on the login form and screen orientation app so protonvpn doesn't look stretched. Having done all that, just upload the mouse toggle, screen orientation and proton vpn apps to your firestick using apps2fire, setup the mouse toggle app (you might need to update it using the app installed on your phone) and lock the screen orientation to landscape and you should be able to use protonvpn on your firestick. Then download the aptoide app store and download the 'Mouse Toggle for Fire TV' and 'Rotate Screen Orientation' apps from it (or just buy them on playstore), the first one is so you can click on the login form using the mouse pointer and the second one is to force the app orientation to landscape so protonvpn doesn't look stretched. Use it for free on FireStick, with 10 GB data a month, and access to 5 servers. ProtonVPN Free VPN for Firestick with great in-app features, and unlimited bandwidth.

Comes with 10 servers on the free app., with basic speeds. On your android phone go to the playstore and download the 'Apps2Fire' app, enable adb debugging and apps from unknown sources on your firestick and connect the app you just downloaded to your firestick, your phone has to be on the same wifi network of the stick (make sure you can upload apps to your firestick from the app). Windscribe Best truly free VPN for Firestick with 10GB free data per month. I know this thread is 1 month old, but i'll leave here how i managed to make ProtonVPN work on my Fire TV Stick 4K in case someone else is having trouble with it.

Protonvpn free firestick