Just use 'Firefox Developer' version desktop or 'Nightly' on android and instal from file (.xpi just a renamed .zip)

[corkiejp]

No need to hack .js files and it stays permanent not required to autoload.

Don't forget above setting in about:config

On android Nightly, go to settings - about and tap on the logo a few times for developer mode and an option to install from file appears.

Edit: Normal Firefox on android can do it as well.