Some steps to try. Between each step, see if Thunderbird works any better. '''1.''' Reboot the computer. '''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 Windows 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] If Thunderbird works OK when Windows is running in Safe Mode with Networking, but doesn't when Windows is running normally, then another process/program is interfering. The first thing to look at in this case is the security software on the computer. That means the Firewall and Antivirus programs. Open them up, tell them to update themselves, both the program and their antivirus definitions. Report back after trying these things.