This post has been replaced for Thunderbird and Windows 10.
I have a number of different mail accounts which I read using Mozilla Thunderbird. Each time a new account is added it appears at the bottom of the mail list. One new account I wanted to move near the top, but Thunderbird does not give an option to reorder accounts. The answer is to edit prefs.js file. On Linux systems
~/.thunderbird/<your profile>/prefs.js, and for Windows
C:Documents and Settings[User Name]Application DataThunderbirdProfiles<your profile>pref.js