This is my favorite dark mode add-on for Firefox (and Chrome) right now, but it still has some things to be desired, such as a time-based dark-mode, and the fact that it can't control the "theme" so the search bar is dark too, and things like that. Consequently this add-on pairs best with the stock Dark theme for Firefox, which is a bit bland to me. Also, this add-on (and the Dark theme) cannot affect the appearance of any sites core to Firefox like the Add-ons page (which makes sense from a privacy and security standpoint, but not from a user experience standpoint). I'd also like to have more control over the colors and adjustments than just sliders or CSS code or Toggle buttons. It would also be nice if it could darken the screen before a site loads because the flash of bright white after clicking a link is quite jolting and unpleasant.
Dark Mode for Chrome (and maybe Firefox?) has a lot more flexibility in that realm, but it's just not as quick or intelligent as Dark Reader.
Also some of my complaints are more at the way Firefox handles add-ons, so there's not a lot that the developers can do about those things.