Do each of these, in order. '''1.''' Double-check the e-mail server settings in Thunderbird with what your e-mail provider says they should be: Comcast / Xfinity https://customer.xfinity.com/help-and-support/internet/email-client-programs-with-xfinity-email/ Thunderbird menu: Tools: Account Settings: For incoming e-mail: Select "Server Settings" on the left side under your e-mail account, and the settings are on the right side. For outgoing e-mail: Select "Outgoing Server (SMTP)" on the left side, then select the server you want on the right side. Click the "Edit" button to change the settings. '''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. Then report back.