Fixed tabs move (for FF17+), and added tabs copy.

This commit fixes the moving of tabs with FireFox 17+, and adds (fixes ?) the possibility to copy tabs through drag'n'drop.

Please, consider adding it to the master branch.

On a side note, I tried to fix the drop indicator animation, but, even though it worked flawlessly with my test profile, it seems to break (at least) one add-on with the profile I use for my day to day browsing (probably "Tab Utilities Lite" or "TabGroups Menu").