That is usually a corrupt installation. My suggestion: # exit thunderbird # download the same or equivalent version from https://www.thunderbird.net/en-US/thunderbird/all/ (that is, if you are with ESR, stay with ESR. if with monthly, stay with monthly) # although not necessary, I always encourage doing a backup prior to a fresh install. #* ensure thunderbird is not running #* enter %appdata% in search window on task bar and press Enter key #* this should open windows file explorer in Appdata\roaming folder #* highlight the Thunderbird folder and copy to external media (or a separate folder) # install the downloaded new version