What exactly do you see in Zotero? Do you see a Zotero Quick Start Guide? If so, what's the Date Added value in the right-hand pane?

Oui, je vois le "guide rapide pour débuter". La date d'ajout est 02/12/2016.

OK, right, so see this explanation of what happened — in your case, in 2016.

You can fix it the same way: by going to the Advanced → Files and Folders pane of the Zotero preferences and setting Zotero to look for the 'zotero' directory in your Firefox profile directory.

When you start Zotero back up, you should see your old data. Once you've done that, you can close Zotero, delete /Users/mathgoupil/Zotero (which should be mostly empty (e.g., with nothing in 'storage'), but don't empty the trash yet, to be safe), and when you start Zotero again it should migrate your data from your Firefox profile to /Users/mathgoupil/Zotero, which is the new default location.

Try pressing Cmd-Shift-G in that dialog and entering ~/Library. (I think the directory is still named "Library" in the filesystem even though it displays as Bibliothèque in the UI.)

Assuming you see the 'zotero' location within your Firefox profile in the Advanced → Files and Folders pane of the Zotero preferences, then /Users/mathgoupil/Zotero is an empty database that's safe to delete — you should be able to confirm this by checking the 'storage' directory, which should be empty. It'd be a good idea to delete that directory to avoid future confusion and to allow Zotero to use that /Users/mathgoupil/Zotero for your real data.

The easiest thing to do here is to close Zotero, rename /Users/mathgoupil/Zotero to /Users/mathgoupil/Zotero-Old, and then restart Zotero. It should automatically migrate your data to /Users/mathgoupil/Zotero. After that you can remove the Zotero-Old directory.