Second, it’s probably that they put their devs on Chrome because it has a larger usershare and Firefox has it’s own VPN service that maybe they don’t want to compete with.
Could possible be because that extension is modeled from the android app and is just a wrapper like a chrome app … I remember years ago I used to run android apps in developer mode using ARChon Runtime so APK’s would run just the same as they did on a tablet and today they have stuff like Elcetron, Node.js etc … My guess is the android app is easy to convert to a chromeapp or chrome extension whereas the firefox or other native applications probably has to be written from the ground up.