Thanks to @Berny, @harlan4096, @andrew75 for you kind replies and support.
A few days have passed and il seems that we have solved this by following quite all your suggestions.
In this case, we have deleted the infected emails inside Thunderbird, even from the trash, and we have compacted the Thunderbird databases.
At the moments, it seems that there was no need to add any exclusions to the antivirus, and there is no need to repair the Thunderbird databases.
Thank You!
Regards,
ABC
