Fix context menus for non-Gecko/non-IE browsers

This fixes a logic error in commit c98271e6a0f0692610e02154029d16a71ecfff2e 

The "contextmenu" event handler should _not_ be added for gecko & IE < 9 browsers. If you look at the original commit, this line had `if (!gecko)`, but the new `captureMiddleClick` flag has the opposite meaning.