Rated 4 out of 5 stars

Nice addon, but I don't like it to overlay the scroll bar.

I used this userstyle (for Stylish https://addons.mozilla.org/en-US/firefox/addon/stylish/), similar to this addon, that perfectly fits my needs:
http://userstyles.org/styles/45050/addon-bar-auto-hide-fast-fade-in-slow-fade-out
May need some tweak, you can do it manually accessing the code from the userstyles list in the addon manager, it's quite easy to tweak, quite intuitive.
But there are other alternatives https://www.google.com/search?hl=en&safe=off&q=hide+addon+bar+site%3Auserstyles.org

Anyway, I was looking for a replacement for AutoHide Statusbar addon for FF versions bellow to 4 (here you have AutoHide Statusbar 0.4.7 http://www.thunderbird-forum.de/p12521-autohidestatusbar.html#post12521 that has been removed here), that works for the addon bar of FF 4 and above. The most similar solution I have is http://userstyles.org/styles/37319/autohide-add-on-bar-fx-4 and specially the modification I posted there, which I'm using now.