User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.9.1.5) Gecko/20091102 Firefox/3.5.5 Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.9.1.5) Gecko/20091204 Thunderbird/3.0 This happend on two different computers: after upgrading to Thunderbird 3 some Accounts appear twice in the folders pane. Both function identically and hold the same messages. When you delete one of them... the other one dissappears, also and all the messages. One envelope icon remains on the folders pane instead. Reproducible: Always Steps to Reproduce: 1. Use W2K PC (German) with several POP3 accounts 2. Upgrade Thunderbird V2.x to V 3.0 Actual Results: Some POP3 accounts appear twice on the folders pane. When you go to the accounts and delete one of the identical accounts, they both are deleted and the folders pane shows one envelope icon which seems to be a placeholder.

I use the English version of Thunderbird.

Problem of bug 303542? Check account/server related entries in prefs.js.

Sorry - that thread ist too complex for discussing this with me. I found prefs.js in \Mozilla Thunderbird\defaults\profile , which is a file with a remark which warns to edit it directly.