The "Application.ini" provided with Thunderbird does not correctly set the path to the icon location. Visa vi it refers to '$out/lib/thunderbird-17.0.11esr', which doesn't exist, but '$out/lib/thunderbird-17.0.11' does. To remedy this problem I added a new variable called 'verName" which adds 'esr' after the variable '$version'. Then change the variables necessary so that the build process sets working references.
Fuuzetsu suggested putting the verName variable inside of mkDerivation. I cannot surmise.
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