With the USE setting 'gtk2', the cut and paste to and from Mozilla-1.0.1 does not work. Then gtk+-2.0.6-r2 is used. With the USE setting '-gtk2', it works fine. Then gtk+-1.2.10-r9 is used.
I bet it's an upstream bug.
Mozilla bug, not Gentoo. GTK2 support in Mozilla is still beta. Closing.