Work around is to download the binary of Seamonkey 2.0.14 from mozilla/seamonky, extract and run from a local directory. To extract the .tar.bz2 file use

tar xvfj seamonkey-2.0.14.tar.bz2

then to add Seamonkey to the menus, add a .desktop file manually in ~/.local/share/applications

[Desktop Entry]
Encoding=UTF-8
Version=1.0
Type=Application
Terminal=false
Exec=/home/username/opt/seamonkey/seamonkey
Name=Seamonkey
Icon=/home/username/opt/seamonkey/chrome/icons/default/default.png
Categories=Network;Internet;Other

then add a symlink to /usr/bin so dmenu picks up seamonkey...

sudo ln -s /home/username/opt/seamonkey/seamonkey /usr/bin/seamonkey