Hi Flood.
I was working on this issue a little bit more, but I was unable to find what is making the update.
I had to act quickly with Thunderbird installation (I reinstalled it from scratch to version 60.0) because it has the flag "auto update" set to yes by default, so as long as you end the installation process it looks for an update and installs it... So I worked offline to avoid it and set the flag to off.
I checked the update app status for Thunderbird in KIS, and it is off. So I agree with you, the issue is NOT KIS.
But at the end, I found version 68.8 and now 68.9 installed and I don't know how.
To avoid to spend more time on this, I just set the flag related to the extension's version control in Thunderbird to NO. So they still work even if the App is being updated...
Not the right solution, I know, but it is working for the time being. Looking for an alternative way of working.
Thank you for your help anyway.
Best regards.