Firefox doesn’t spell‑check in Copilot because Copilot uses a modern custom text editor that Firefox doesn’t recognize as a normal typing area. Since Firefox never turns its spell‑checker on in this kind of editor, nothing actually fails, so there’s no error code or message. The spell‑checker engine simply never activates. For this to work, Firefox would need to update its spell‑checker so it can handle newer web‑based editors instead of only traditional text boxes.