To be fair some things can’t be polyfilled, like the WebUSB API, as someone mentioned in the comments. But lots of the sites could fix their code so it works on Firefox, yeah.

But you can’t make a native app think it is opening Chrome when it’s opening Firefox with just an extension. The extension only changes things inside the browser, but the Canon app probably checks the app ID of the browser it’s opening, which is done at the level of the Android system, and which is not affected by the extension.

Isn’t Mulch not maintained anymore, as it was maintained by DivestOS, which has shutdown?

The distro choice isn’t that difficult, as if you have experience with Linux you know what you need, and if you don’t, just choose any beginner-friendly distro (probably Ubuntu flavors and Mint).