Hi Silvain, could you open the Customize tab and see whether the "Bookmarks Toolbar Items" control somehow moved from the Bookmarks Toolbar to the main bar. You can drag it back.

See: Customize Firefox controls, buttons and toolbars

This screenshot isn't exactly for your situation, but it shows what the control looks like:

Unfortunately, I do not have such item in my favorite tools. Dragging any of my bookmarks into the toolbar is impossible. Except one item I could drag, the Display bookarks icon (and now it is there I cannot move it back to the toolbar. Is there a way to find it through about:config, in case it was disabled there? I join a printscreen of the tools/items availble for the toolbar (names are in French!)

Can you find the "Bookmarks Toolbar Items" (Barre personnelle items?) on your main toolbar and move it back to the Bookmarks Toolbar?

If needed, you can use the "Configuration par défaut" button at the bottom to put the toolbar buttons and controls back into their original positions and then re-customize the bars.