Rated 4 out of 5 stars
Great add-on, unfortunately I'm missing two features:
1. Open DM in background.
2. Close DM after download is completed.
Also, I'm using userstyle (http://userstyles.org/styles/45538/firefox-4-in-content-tabbed-download-manager) and nav bar is still there after first opening. I have to switch to another tab to hide it. It's working OK until a restart Firefox again.
I didn't check the code for those two specifically, but I think I made both to follow the preferences that exist for those things on the normal download manager, so actually both of those should already work if you switch the correct prefs, which you can find in normal Firefox download manager docs.
No idea about userstyle, but usually, it should only show the nav bar the very first time you open it and then always, even after a restart, show without it. The very first launch is unfortunately a necessity with the current infrastructure, but that may improve in future Firefox versions.