Text generation within AI models is a known limitation, and Firefly is no exception. Adding text or symbols directly during the generation step can sometimes be unreliable, even when quoted.
One good option is to generate the image without the text/symbol and then add it afterward in Adobe Express.
Another workaround you might find helpful is using Edit in Boards. Load your generated image with the unwanted text, choose Edit, then Edit text in image, and you’ll be able to remove it.
Fix text in Boards
I hope these steps help you achieve the result you’re looking for. Please let us know how it goes.