Fix context menu in firefox
Fix context menu in firefox

Fixes #721

Tyriar/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js,sourcelair/xterm.js,Tyriar/xterm.js,xtermjs/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,sourcelair/xterm.js,xtermjs/xterm.js,Tyriar/xterm.js,Tyriar/xterm.js,akalipetis/xterm.js,akalipetis/xterm.js,xtermjs/xterm.js,xtermjs/xterm.js,akalipetis/xterm.js