to install permanently: create a zip file with a ".xpi" extension, containing: manifest.json mdn/inject1.css mdn/inject2.css e.g. `zip mdn-css.xpi manifest.json mdn/inject1.css mdn/inject2.css`
to install permanently: create a zip file with a ".xpi" extension, containing: manifest.json mdn/inject1.css mdn/inject2.css e.g. `zip mdn-css.xpi manifest.json mdn/inject1.css mdn/inject2.css`