Yeah, Firefox completely changed their sidebar theming when they added vertical tabs, so for better or worse that's the style nowadays. I agree with you that it's busier than I would like (and I feel that way about Firefox's built-in sidebars too), but I want Tab Stash to feel as close to "built-in to Firefox" as I can manage. "Stays out of the way" is exactly what I'm going for!

If I hear from more folks (preferably on GitHub, where it's easier to have a back-and-forth conversation), I'll certainly consider trying to find a better balance between "makes good use of space / is clean" and "looks like Firefox".

In any case, thanks for the review, and I'm glad you're enjoying it!