Your issue likely applies to any add-on you are trying to install on Mozilla Focus since it does not support installing add-ons from the browser, which makes sense since Focus is a product whose primary purpose is to ensure the highest data privacy and security: allowing any add-on created by random people would be a massive potential for data breach and hacking. Most people use Firefox Desktop or Firefox for Android, and add-ons work fine there. There has been a recent upgrade from Mozilla that might allow you to install the .xpi though: https://bugzilla.mozilla.org/show_bug.cgi?id=1814123. Feel free to open an issue in our Github repo if you'd like to discuss more, but again, we never claimed that this add-on would work on a Firefox product that isn't suited for add-ons.