While browsing through some of the experimental themes, I happened to run across another one of the best dark themes I have seen in a while. LumiNight has a very eye appealing look and feeling to it, as you can see here bellow.
Here is a little more from the developer:
The goal of LumiNight is to be modern & dark, with icons that can be understood easily at a glance. I went into it trying to avoid gloss-overkill and shiny orbs. This theme tries to be as straight-forward as possible, while appealing to those that like a darker theme.
This theme is brought to us by the same great designer who brought us the MushroomKingdom theme from a few weeks ago. Personally, I hope they keep up the great work! You can pick it up on the Firefox Add-ons site. Now the only question I have though is when will we see LumiDay?