Recently I switched from KMail to Thunderbird for various reasons. A little thing that I didn't like in Thunderbird is that in the date column it displayed dates as MM/DD/YYYY. However I have my system locale set to DD/MM/YYYY. I searched Thunderbird's bugzilla and found a really old bug from 2004 which is still open! So I don't expect an upstream fix soon. Afterwards I searched if there is an extension which changes the date format but I didn't find something.Fortunatelly I found a solution. Here is how Thunderbird looked before my change.
In order to change the date format in the Date column open from Dash Main Menu > Internet. Select Thunderbird from the list and press the Properties button. In the command line edit type:
export LC_TIME=en_GB.UTF-8 && thunderbird %u
Again open KDE Menu Editor > Internet > Thunderbird or edit directly the Thunderbird's launcher. In the command line edit type:
That's it. Now Thunderbird will correctly display the date format in the Date column.
In the same way you can change the command with which you run Thunderbird in any Desktop Environment. I have tried this with Thunderbird 13 and a couple of older versions and it works without any problem.