Display favicon in shortcut button?

Good morning. I'm trying to configure my shortcuts on my home screen. Specifically, I want the shortcut button's icon to display the website's favicon, rather than just the default circle with a letter. I set the custom icon field for the button (as an example) to: https://www.notationtraining.com/favicon.ico I have verified that this favicon exists and I can load it into a separate firefox tab. However, it won't display in the shortcut button. I've tried restarting and trashing some caches without success. Where am I going wrong? I'm on FF 151.0.4 for Mac OSX 26.4. Thanks, Eric