Not sure where you are downloading from or which version. It may be useful if you post that info. Have you uninstalled previous version (if no then uninstall it) or is this a first time use of Thunderbird? Do you have any AV software scanning the download? If problem persists try switching off AV until you have downloaded and installed. Try this link: http://ftp.mozilla.org/pub/thunderbird/releases/52.9.1/mac/ then select language eg: en-GB = English British. Then download the 'Thunderbird 52.9.1.dmg' file to computer. Run that file to install.