''re :I keep some active mails "open" in Thunderbird'' Do you mean you opened them using a double click so they are opened and readible in various Tabs in Thunderbird window? When you exit and later restart Thunderbird, it should have remembered those Tabs are open, but if you have software on your computer that can 'clean up' files, eg: CCleaner, WiseCleaner or some Anti-Virus will also 'clean up' files then it's possible those types of program deleted the session files, so Thunderbird has no idea what was previously opened in a tab. Please make sure the Thunderbird profile folder is exempt from being accessed by those programs.