The thing is, Firefox/Gecko isn't embeddable (probably the one of worst tech blunders ever). I wonder if Tauri could wrap around Blink, instead? Then your app could just ask for Chrome to be installed.
The modern Qt web view component, QtWebEngine, is actually Chromium-based. So you could imagine a Tauri which uses Qt instead of GTK and uses QtWebEngine as its renderer instead of WebKitGTK.
