If steps 1-6 don't work you might need to create an entirely new menu item. Now remove old icon in Menu panel, and in the Menu favorites list:
In the Launcher Properties box, click the Firefox icon on the left.In right pane, click Firefox Web Browser > click Properties.Took me forty-five minutes and some help from a fellow at the LinuxMint forum to figure something out what I should have known. Sometimes the easy answer is staring you (or in this case 'me') right in the face.
