It's possible too many connections are being cached. This is a restriction enforced notable on email providers like GMail. # Go to '''Edit > Account Settings > Server Settings > Advanced > Maximum Number of server connections to cache''' for the problematic account # If it's set to 5 set it to 4 or lower. # If you have several devices (phone, tablet, other PCs, etc) connecting from the same location, try to set this number to a combined total of 4. This is a per-IP limitation of some servers If this doesn't help, in the interest of getting you back to a normal working setup rapidly, you can go back to your previous Thunderbird version. In Windows Thunderbird updates are automatic by default. To avoid similar situations in the future, you could change that and decide to have a more conservative approach to upgrades. Before trying anything, [https://support.mozilla.org/en-US/kb/profiles-tb#w_backing-up-a-profile make a backup of your current Thunderbird profile]. You can uninstall the current Thunderbird version via '''Control Panel > Programs and Features''' and reinstall a previous version, download it from here: * [http://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/31.0/ Version 31.0] * [http://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/24.8.0/ Version 24.8] Click on the type for your OS. eg: If using Windows select 'win32' then select the language eg: en-GB = English british then download the .exe file You can [https://support.mozilla.org/en-US/kb/configuration-options-updates?esab=a&s=automatic+updates&r=1&as=s customize automatic updates behavior], however, it's under '''Tools > Options > Advanced > Update'''. My suggestion would be to set it to '''Check for updates, but let me choose whether to install them.''', specially if you depend on email for critical / important business or personal matters.