As you know, SeaMonkey version supplied with Ubuntu 9.10 is still v1.1.17 which is outdated.

To install SeaMonkey 2.0.1 simply add this line to your system’s software sources:
using the menu: System > Administration > Software Sources > Third-Party Software
or by editing this file: /etc/apt/sources.list

Ubuntu 9.10:

previous Ubuntu versions:
deb karmic main
deb-src karmic main

Then install SeaMonkey 2 by typing these commands in a terminal:
sudo apt-get update
sudo apt-get install seamonkey-2.0

That’s it!

SeaMonkey 2 will install alongside SeaMonkey 1.x and will import all your e-mails, passwords, bookmarks and preferences and it works on both 32 and 64-bit systems.

By default, a shortcut to the e-mail client won’t be created, you can create a new one (or edit the one used for SM1) and point it to the command: seamonkey-2.0 -mail

Thanks to Joe Lesko for making it possible!

And if you want to find back the good old Netscape look removed from SM 2.0: Install the Classic Default theme: (attention, this theme is still in beta state).