Change: _preventNextContextMenuEvent now does not check the event
initiator.

It checked for `MouseEvent.button == 2` before. This check was removed as
there is a chance that Mac OS Firefox does not set this event property
properly (might be using MouseEvent.which on that platform).
Either way this was unnecessarily tight even before this.
Change: RMB activation key can be now used on Mac OS.
Change: abandoning querySelector* in favour of xpath because compatibility

document.querySelector and document.querySelectorAll are available since
Fx3.5. The document.evaluate is available since Fx1.5 and is the engine of
the new methods:
+ new method _xpathSelector replacing document.querySelector
+ new method _xpathSelectorAll replacing document.querySelectorAll

+ minor doc comment change