This is because you've downloaded the "main" branch. You need to download from the release page here : "https://github.com/YouG-o/YouTube-No-Translation/releases/tag/v2.23.0" in the "Assets" section you can find the Firefox zip.

EDIT : v2.23.0 is available on Mozilla. If you installed it via Github, I highly recommend you to re-install it from here, to get automatic updates.