Thanks for the kind review. I'm glad you found Tab Stash useful!

Regarding stashing multiple tabs, one quick way to do this is to use Shift+Click to select a range of tabs in your Firefox tab bar. Then click the "Stash" button in the Firefox toolbar. It's not quite a context menu, but hopefully this saves you a few clicks if you were doing it one-by-one before.

Dark mode is a pretty common request—I haven't had a chance to investigate this in the most recent Firefox release, but at least up through Firefox 69 it wasn't possible to detect the browser theme, so I haven't been able to implement it so far. Once Firefox provides this ability to add-ons, it's definitely something I want to revisit.