The all-powerful about:config in Thunderbird and Mozilla other offerings

http://kb.mozillazine.org/About:config

I will definitely plan on reading more on this. Other email clients offer the same customizations. Outlook has its own version of about:config, but it requires intimate knowledge of programming such as VBScript. Thunderbird uses XML which at its core is simply a plain text file. The XML parser then takes this XML file and put the settings into effect. Therefore, as long as you can understand XML syntax which is not that difficult to learn, you can be quite a "programmer" for software packages that use XML as its configuration base. I use VMware Player. I am not sure if it uses XML, but its configuration is just a plain text file as well.

In case you want to see what is on this file, it can be found here
If the link is broken, here is a copy.