As you may know, SeaMonkey is an internet suite that includes a web browser, an email and newsgroup client, a HTML editor, an IRC chat and web development tools.
The latest version available is SeaMonkey 2.49.4, which has been released a while ago, coming with changes and new features. Unfortunately, SeaMonkey is not maintained by Mozilla anymore, but it is still an awesome suite, stable enough to be used by *nix enthusiasts.
Due to the fact that it is available via the Ubuntuzilla repository, installing it on Ubuntu 18.10 Cosmic Cuttlefish, Ubuntu 18.04 Bionic Beaver, Linux Mint 19.x, Linux Mint 18.x, Elementary OS 0.5 Juno and other repo-compatible systems should not cause too many problems.
All you need to do is add the repository and the key to your system, update the local repo index and install the required package:
$ echo -e "deb http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt all main" | sudo tee -a /etc/apt/sources.list > /dev/null
$ sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com C1289A29
$ sudo apt-get update
$ sudo apt-get install seamonkey-mozilla-build
Optional, to remove seamonkey, do:
$ sudo apt-get remove seamonkey-mozilla-build