In my opinion, this is a MUST-HAVE for anyone who wants to reclaim screen space. Getting rid of the Menu Bar helps a lot, but I don't want to get rid of my Add-On Bar, especially as I use ABP, Ghostery, and FlashBlock, all of which I need to be able to modify per-website. This allows me to do just that, but opens up the bottom of the screen giving a much more streamlined interface.
Simple, elegant, opens screen space, can be pulled up or down to give even more space, have not run into any program-based bugs
Works wonderfully on every screen I have, including 55" LED-LCD 1080p 240hz Television (hooked up to computer via DVI or HDMI), my gaming and HD Video/Audio Editing PC's 30" 2560x1440p I-IPS and 2x 24" 1920x1200 screens, to my laptop's 18.4" 1920x1200 screen...
Haven't tried to use it with FF Mobile (android) yet, as FF-Mobile itself sucks.
However, this add-on ROCKS