# first, you need to do a backup. I presume the profile is at c:\<yourid>\appdata\roaming\thunderbird You can verify that by clicking help>troubleshootinginformation, scrolling to 'profile folder' and clicking 'open folder'. I place sample backup unstructions further down in this post # down latest thunderbird from https://www.thunderbird.net/en-US/thunderbird/all/ Select the ESR version 140. That is updated annually. # uninstall the beta. if prompted about data records, select to leave them. # now install the ESR release. If there are issues, post back on this thread for assistance. As soon as possible, click settings>general and set the update feature to advise but NOT to automatically install updates. I recommend that, as you are using Thunderbird as a business and will want to control software updates. Sample backup steps are below: ''BACKUP''' # 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 to a separate folder Whether this resolves your issue, I have no idea. My presumption is that your tax software is desiged to work with current releases of email software. Good luck to you in resolving this.