This style is a quick way to autohide (reappearing on hover) the status bar. It leaves a 1px border to trigger :hover (the default color is ButtonShadow, which looks fine on a native-colored theme on Windows).
Useful companion extensions to a hidden status bar: "Fission" for progress in the address bar, "Handy Xtra Stuff" for displaying attribute values in tooltips
I needed this because I discovered that the Greasemonkey button for the Custom Buttons² extension required the GM status menu, and the status bar it's on, to not be hidden; which defeats the purpose of moving status buttons into the toolbar in order to replace them. Now, the status bar is out of the way, but quickly accessible, and all its menus can be appropriated as necessary (I'm looking forward to breaking some things with UserChrome.js in the same way).
(In the end, I added code to the button move the popup menu to the toolbar so I can hide the status bar properly.)