Initialize after mouseover on #PanelUI-popup:
we doesn't receive mouse events from #PanelUI-button anymore, at least in Firefox 31.0a1 (2014-04-27)
(#7)
Remove mouseover listener, if initPopup() called by another trigger
(#7)
Initialize by mouseover after small delay to improve performance
(#7)
Refactor: add destroySbBtnWatcher() function
(#7)
Correct changelog: we already have sidebars button in Firefox 30.0a2
(#7)