Safe Mode is a special Thunderbird execution mode that can be used to troubleshoot issues in Thunderbird. In Safe Mode, you can reset some settings or disable add-ons that might be the source of problems. By comparing Thunderbird's behavior in normal mode to its behavior in Safe Mode, you can find out if problems are being caused by a specific add-on.
To confirm that Thunderbird is running in Safe Mode, click the Tools menu and then click Add-ons and Extensions. Above your list of add-ons, there will be a banner displayed stating that Thunderbird is in Safe Mode.
Based on information from Safe mode (mozillaZine KB)