as of xulrunner 1.9.2, conkeror's element/frame focus tracking is no longer necessary.  What is necessary
is to focus the xul:browser when we exit the minibuffer or kill a buffer.  Note that the element/frame
focus tracking system must be retained for compatibility with older versions of xulrunner.  That code can
be removed if we ever drop support for xulrunner 1.9.1.