use capture mode to detect event in content area
fx33: rename abbreviated flag 'disableMCB' to clearer 'allowMixedContent'

* https://bugzilla.mozilla.org/show_bug.cgi?id=1038604
fx33: follow extra dispatching 'SwapDocShells' event

* https://bugzilla.mozilla.org/show_bug.cgi?id=1019990
make setting list for click actions
support click action on background tab
set inactive state values to null
optimize flags for observing clicks
scroll overflowed tabbar by mouse wheeling on scroll buttons