Thanks for the detailed feedback, and sorry about the rough start!

The problem where subtitles stay untranslated until you change a setting should be fixed. The extension used to attach to the wrong caption container on a fresh tab, and changing a setting just forced it to try again. It now re-attaches on its own when you open a new video, navigate within YouTube, or come back to the tab. There was also a separate fix for Firefox specifically: requests to Google had stopped going through outside Chrome, which left the tooltip empty. Both fixes are in the current version, so please update the add-on and refresh the YouTube page. If you still hit it, write me at hovertranslate@gmail.com with your Firefox version and a video link, and I'll dig into it.

About English-to-English: that's not possible right now, because the extension uses translation services (Google, Bing and now DeepL), and they translate between languages rather than explain a word. Showing definitions instead of a plain translation is something I really want to add, and your review is another vote for it.

Thanks again - this kind of report is what makes the extension better!