The official websites that provide downloads of Firefox and Thunderbird only offer the latest version of the software programs. This is done to protect the users from downloading outdated, insecure or unstable versions of a software.

It is therefore, usually, not a good idea to download and install an old version. It might be different during beta testing but it is generally recommended to figure out other ways of coping with a problem.

There is however a very easy way of downloading old versions of either Thunderbird or Firefox.

Just point your web browser to ftp.mozilla.org and visit the ftp repositories for either program. The ftp server not only lists the latest version but also all other releases that have been created in the past. Here are the direct links:

Download: Older releases of Thunderbird
Download: Older releases of FireFox

Caution: It can be highly problematic to install old releases. Make sure you backup the data before installing an older version of Thunderbird or Firefox.