This works seamlessly. And has a nice idea where to put the annotation button, in comparison to other addons. A frequently used function will now need less round trips through the bookmarks to find the bookmarklet. Many thanks!

Additionally, it may be interesting to learn how and why this extension is different than the fork from the Chrome extension that lives at https://addons.mozilla.org/en-US/firefox/addon/unofficial-hypothesis/

Works without hussles, managed to installed it also on Android Firefox-nightly following the official-procedure for custom-collections: More tips: http://blog.mozilla.org/addons/2020/09/29/expanded-extension-support-in-firefox-for-android-nightly/.