dCore-seamonkey-installer is now availble for 64 bit dCore. When the web browser brings up the Seamonkey download page, one has to go to the Contributed Builds section, and then the Linux/x86_64, and under that download the Linux/x86_64.tar.bz2 file. This is also mentioned in the dialogue.
I have tested the resulting SCE with only xorg-all, icewm, dCore-seamonkey-installer, and seamonkey on dCore-stretch64. All works as expected.