If you are restarting thunderbird in Thunderbird Safe Mode Help > restart with addons disabled and it works ok, then there is an addon which is causing a conflict. Tools > Add-ons so disable all addons then restart Thunderbird and it will start in normal mode. Gradually enable each addon extension and test. when it fails, the last addon you enabled is causing the problem. The addon may need updating. If you mean you rebooted the computer in safe mode and Thunderbird opened ok, then there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.