See http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

Well, thank you very much, but unfortunately, these informations seem to be for PC users while I'm using a Mac… The few indications concerning mac users doesn't work (the file does'nt exist!)

Make sure that you look in the correct location for the Firefox Profile Folder

Help > Troubleshooting Information > Open Containing folder - See also Use the Troubleshooting Information page to help fix Firefox issues

Mac: /Users/<user>/Library/Application Support/Firefox/Profiles/<profile>/

The problem is that this file doesn't exist (on my computer)! I've tried to download and instal a new version of firefox with the same result. :-(