Best Speed/Fast-Dial addon, and I guess I tried 'em all over time.

This one relies on non-commercial, non-proprietary tech and is transparent because you can handle the bookmarks in the standard Firefox bookmarks view.

Thumbnail refresh is rather slow, but I this might be a limitation of webextensions (it has to load the whole page in a new tab first).

Let's hope this one keeps being maintained - I'd like a "dark theme" with plain dark background.