Reading this answer I found that á and è (and also right now in this question) characters from Déjà Dup are shown different than the other characters (they are less "bold" than the others):

Link: Why does Déjà Dup say there is no space left?

Is this my problem with Firefox 21 in Ubuntu 12.04.2 LTS or is a problem of the web (I don't know if the previous design had this problem)?