Thank you @Berny for welcoming me in this forum, I read the 2 links you gave me and did all the suggested actions. The issue was still there. For a few minutes I thought I found the culprit (3 shortcuts on the Firefox home page which pointed to torrent websites). I removed the shortcuts and the problem was gone. But when I put the 3 shortcuts back on the Firefox home page, the issue did not occur any more. I do not know what to say. Now that I cannot reproduce the issue, the investigation is over. I just have to wait for it to come back. Anyway, thank you for your help, and happy Christmas Eve.