To install Thunderbird, you can either

#1) go to a command prompt and type "apt-get install thunderbird"

or

#2) go to system->administration->synaptic package manager, search for
thunderbird, select it, right click to "mark for installation" and then
click the "apply" button.

Personally, I'm liking Thunderbird 3, but the default is Thunderbird 2
as 3 is still in beta.