Please do each of these, in order. '''1.''' Run a Check Disk to fix any file system errors. [http://brucejohnson.ca/CheckDisk.html How to run a "Check Disk" in Windows] '''2.''' Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender. '''3.''' Restart the operating system (Windows or OSX), in [https://en.wikipedia.org/wiki/Safe_mode Safe Mode with Networking]. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: [https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Windows 10], [https://support.microsoft.com/en-us/help/17076/windows-8-startup-settings-safe-mode Windows 8], [https://support.microsoft.com/en-us/help/17419/ Windows 7, Windows Vista], [https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/boot_failsafe.mspx?mfr=true Windows XP], [https://support.apple.com/en-ca/HT201262 OSX]. Then see if Thunderbird works better. If it does, then another program or process is interfering with Thunderbird when Windows is running normally. Then report back.