Steps to Reproduce (with Thunderbird 3.0.2): 1. Set Thunderbird to open messages in a new window (which should disable most ways to accidentally open a tab) 2. Open about:config and set mailnews.start_page_override.mstone to 3.0 (bonus points for also setting mail.tabs.autoHide to true in about:config) 3. Close and reopen Thunderbird Expected result: The tab bar remains hidden (I've explicitly told Thunderbird to do so). Actual result: It is shown nonetheless. Work-around: Manually set mailnews.start_page_override.mstone to "ignore" (as with bug 548619 and bug 548620).