I think this isn't a bug, it happens when you drag the tab away from the top. "Easy Organization Get your sites in order. Simply arrange the order of your tabs by dragging them in one swift move of your mouse. Or, if your browser window is getting cluttered with too many tabs, just drag a tab out of its existing spot and it will automatically open up in a new window" From Firefox's website Not sure about stopping it though, maybe having only one tab open can stop it.