We have covered a Thunderbird add-on in the past to change the email folder sort order in the mail client which is handy to move the important folders to the top, and the unimportant ones to the bottom of the email folder listing.
Folders were still restricted by the email account order in the folder listing, with no apparent option to change this in Thunderbird.
Enter the Thunderbird add-on Folderpane Tools. The add-on is compatible with all Thunderbird 2 and Thunderbird 3 releases. It is installed like any other Thunderbird extension. Head over to the official Folderpane Tools page at Mozilla.org and download the extension to the local computer system.
Fire up Thunderbird and open the add-on manager by clicking on Tools > Add-ons in the menubar. Locate the install button, click it and select the downloaded xpi file to install the Folderpane Tools extension.

The add-on offers two additional configuration options. It can override the startup folder, which is handy if a different folder should be displayed on the startup of Thunderbird.
The second option is to rearrange accounts. This is done by selecting an account in the listing and selecting the move up or move down button afterwards to move it to a new location. It is furthermore possible to select a new default account.
Folderpane Tools adds a missing feature to Thunderbird, and is therefor a recommended add-on for users who want to rearrange the account order in the email client. (via Caschy)
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Works great. Found this about one year ago and like it a lot, since I use an IMAP-Account more often and had to scroll down all the time. As you said, recommended!