I don't know how many times I have read this, but for some reason most people go looking at download sites for something that Mozilla has on their site.
They don't advertise it, and actually have a disclaimer that it is for archival purposes and they don't recommend people use it. But every version of Thunderbird ever released, from 0.1 to date are all on this FTP site. This is particularly good for those moving from Version 1 to later versions, because they must install Version 2 to get the necessary conversions to their profile for Versions 3 and latter to install without errors. The version 3 and latter upgrade process does not know how to read V1 profiles.
For those not familiar with FTP sites, they function as a file system so, clicking a link opens the folder below and to go up a folder level you click the up to higher level directory.
In most of the folders in this archive, first you click on the version, this opens a list of operating systems. Select your operating system and a list of languages by code will appear, eg EN_US is a US localised English version, fr is a french etc. Select you preferred language and you will be presented with two files. For windows users the large file with the EXE on the end is the one you need to download and install. You can safely ignore warning from your browser or anti virus programs about it being an executable file, because that is what it is.
Once the file is downloaded, open the file to install Thunderbird.
22 May 2016 : Mozilla have decommissioned FTP on FTP.mozilla.org from the 5th Ausugt 2015. See their blog post announcing the change here.
Thanks to Andy Plotkin for making me aware this link was out of date.