As you suggested, I have uninstalled Greasmonkey, restarted Firefox and reinstalled Greasemonkey: same result (the menu entry "New user script" has no effect) !
On another computer I installed Greasemonkey on a fresh Firefox and all is ok : it seems effectively to be a problem with my profile ... Do you have any idea to solve it ? -- You received this message because you are subscribed to the Google Groups "greasemonkey-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to greasemonkey-users+unsubscr...@googlegroups.com. To post to this group, send email to greasemonkey-users@googlegroups.com. Visit this group at https://groups.google.com/group/greasemonkey-users. For more options, visit https://groups.google.com/d/optout.