Also, I filed a bug, but I don't know if anyone will have interest in working on it since printing often is last in line for fixing, especially when the causes of the problem are uncommon: https://bugzilla.mozilla.org/show_bug.cgi?id=1512893

Until then, the workaround: https://support.mozilla.org/questions/1243091#answer-1179748

Ok, I really thank you jscher2000. I tried the new script and it works well, I also tried it in some other site from which I usually print different articles and it works: finally the margins are the right ones! I hope, however, that soon this bug will be permanently resolved in one of the upcoming Firefox releases. I must admit that when I contacted Firefox support I expected to get some answers about my problem, but I thought it would be complicated (not to say impossible) to solve it: instead I found a kind, helpful and smart person who has dedicated time and work to solve a bug of a function of Firefox that, perhaps, for those who print rarely is not such an important thing, but for me, which I print with a certain frequency, is one of the features that I want to be made in the best possible way in the browser I use. Firefox support is great, and you've been great! Thank you!