To fix the latest issue with Thunderbird and Spamfighter please do the following:
Update your Thunderbird version to 91.6.0
Do not uninstall your current Spamfighter version, but install the following fix on top of your current version:
Click on the link below and install the fix:
http://redir.download.spamfighter.com/spamfighter/Thunderbird/SPAMfighter_TB_7.6.180.exe
Installation takes a few seconds and afterwards your Spamfighter will be working like normal again
Thunderbird is sending out updates regularly which often create issues, but this should solve the latest.