You can use the Ubuntuzilla repository https://sourceforge.net/p/ubuntuzilla/, which repackages the Mozilla builds as .deb files and puts them in a repository so it's easy to install and upgrade.
To add their repository to your system and install SeaMonkey from it, run these 5 commands in a terminal:
sudo gpg --list-keys
sudo gpg --no-default-keyring --keyring /usr/share/keyrings/ubuntuzilla.gpg --keyserver keyserver.ubuntu.com --recv-keys 2667CA5C
echo 'deb [signed-by=/usr/share/keyrings/ubuntuzilla.gpg] https://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main' | sudo tee /etc/apt/sources.list.d/ubuntuzilla.list
apt update
apt install seamonkey-mozilla-build
info from
