This may be a problem with updating prefs.js file. Any change made is immediately effective, but if thunderbird closes without updating prefs.js, then changes are lost.It might be that the file is in read-only state, or an antivirus program is interfering. A way to check is to look in windows file explorer at the profile to see the date that prefs.js was last updated. and sometimes there may be multiple copies of prefs.js, such as prefs-1.js, prefs-2.js, etc. If you see that, I suggest deleting all except the one with highest number and rename that to prefs.js. Do this while thunderbird is not running. Even if that works, the bigger question is what is interfering with updates. What I wrote here may not be the problem, but it definitely sounds like it.

Thank you for the quick response! I have noticed that there is no prefs.js file in the Thunderbird folder. With the help of ChatGPT, I created a new one and tried various things. I also tried to run as administrator, but the signature keeps disappearing anyway.'

david said

This may be a problem with updating prefs.js file. Any change made is immediately effective, but if thunderbird closes without updating prefs.js, then changes are lost.It might be that the file is in read-only state, or an antivirus program is interfering. A way to check is to look in windows file explorer at the profile to see the date that prefs.js was last updated. and sometimes there may be multiple copies of prefs.js, such as prefs-1.js, prefs-2.js, etc. If you see that, I suggest deleting all except the one with highest number and rename that to prefs.js. Do this while thunderbird is not running. Even if that works, the bigger question is what is interfering with updates. What I wrote here may not be the problem, but it definitely sounds like it.