Cedric
Just to be clear, you can do it without advanced search! By just typing the first name in the "recherche" field you will get records for that first name, no need to click on advanced search at all!
For example. see below - I simply typed "gill" into the search box and see the result - only 4 of the usernames actually contain "gill" but all the firstnames and some of the lastnames contains "gill" - they are all brought up
If you wanted the search to
ONLY return first names ever then I guess you will need to go to the CB Fields Manager and mark all the other fields as "Searchable" = NO except the First Name Field - that should do the trick!
Cliff