I can't use button Y when copy Ctrl + C + Y

The most likely reason for this is that there is more than one "Y" hotkey (AKA accesskey) on the menu. Firefox 159 now assigns accesskeys to add-ons. If you press Y a few times, can you see Firefox cycling between multiple menu items? To activate a selected menu item, you can press Enter, but that isn't very convenient. The developers are aware of these conflicts and work is under way to resolve it, for example, by assigning numbers to add-on entries rather than letters. But that might take a couple months to arrive. In the meantime, how important is the add-on entry that uses Y? Maybe it can be removed without disabling the extension, or maybe the author can make that possible soon, or perhaps you prefer to disable the extension. Related threads with the same problem: * https://support.mozilla.org/questions/1615034 * https://support.mozilla.org/questions/1615586