- Preferences -> Display -> Advanced (tab not button)
- If the option "Automatically mark messages as read" has the suboptions "Immediately on display" and "After .... seconds", you can uncheck the checkbox. It won't mark new mails as read anymore.
for older thunderbird versions:
- menu -> "preferences" -> "preferences" -> "Display" -> "Advanced" tab
- uncheck "Automatically mark messages as read".
- close settings
- to mark emails as read click the tiny green circle in the list view
- or press the hotkey M
Edit -> Preferences ->Account Settings -> Junk Settings -> disable: Enable adaptive junk mail controls for this account