@OldSchool40: I too used Seamonkey, for some time, as my primary browser on Windows before coming aboard to Linux, and eventually Ubuntu MATE - Welcome!.
These are the four easy steps which I use; go to your terminal and issue these commands (just copy/paste only what is in bold italics)
echo -e “\ndeb http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main” | sudo tee -a /etc/apt/sources.list > /dev/null
(press enter)
sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com C1289A29
(press enter)
sudo apt-get update
(press enter)
sudo apt-get install seamonkey-mozilla-build
(press enter)
Enjoy!