re : Now mozilla just updated to 60. and VTiger CRM isn't supported. https://www.thunderbird.net/en-US/thunderbird/60.0/releasenotes/ ''IMPORTANT: Add-ons not marked as compatible with Thunderbird 60 by their authors will be disabled (this can be reverted via preference '''extensions.strictCompatibility''')'' Located: 'Menu icon' > 'Options' > ' Options' > 'Advanced' > 'General' tab click on 'Config Editor button' If you want to revert back to an older version of Thunderbird: Uninstall Thunderbird via Control Panel > Programs and Features. Do not use a product like CCleaner as it can delete the profile as well. Note : version 56 was only released as a beta, are you sure it was that version ? Version 52.9.1 was the official release prior to version 60. Install prefered version http://ftp.mozilla.org/pub/thunderbird/releases/ Select version number select 'win32' if using a Windows OS Select language preference eg: en-GB is English (British) Download the Thunderbird setup exe file and run to install. If you want to stop Thunderbird performing automatic etc updates: 'Menu icon' > 'Options' > ' Options' > 'Advanced' > 'Update' tab There are three options. If you want to stop the updates then select the third option: 'Never check for updates (not recommended: security risk) click on 'OK' It might be worth contacting VTiger CRM asking them to make a product plugin/extension compatible with current versions of Thunderbird.