While using Thunderbird for Windows or Mac, you may find that messages are still marked unread even after you have opened them. This is because Thunderbird does not consider the message as “read” unless it has been open for five seconds. You can change this setting with the following steps.
Thunderbird for Windows
1. Click on Tools and select Options.
2. Click on Display and select Advanced.
3. Select “Immediately on display”.
Thunderbird for Mac
1. Click on Thunderbird and select Preferences.
2. Click on Display and select the Advanced tab.