It is just you if its not appearing.

I'm using Ad Nauseum which is just UBO but improved with added features and it appears just fine.

I've got uBO on my Android Firefox and the form is visible. UMatrix shows only first party css, no js at all, which is good. You might have other extensions. View source on my phone shows a very simple HTML page with a form that posts to list-manage.com Maybe something added to your Firefox is blocking that.