Thank you for this great feedback. The not being able to scroll seems to be a newer bug with Firefox and only if you try to do it via the options menu. If you use the extension icon bar in the top right and add it this way it worked. Either way I have now reworked the select input to always have a scrollbar and also a search.
A "smart selector" would be nice to have but would be quite a bit more work so it's not included yet.