Rated 5 out of 5 stars
Perfect for being able to auto-hide the tab bar when there's only a single tab without losing the Firefox button.
However, this icon doesn't show whether I'm in private browsing or not. It would be nice if the icon changed when I'm in private browsing. Also, modern versions of Firefox display a mask icon next to the Firefox button when in private browsing, which this addon should override.
Maybe the Firefox icon this addon displays should be replaced with a rectangle saying [Firefox] just like native Firefox does (with or without a mask icon), so that Firefox with this addon looks exactly like without it unless you move the icon. (maybe as an option?)
The mask icon is available in the newer versions of Firefox.