It's disabled by default, but I had no idea this was added.
Firefox mobile users started noticing something peculiar this week when sharing links to WhatsApp. After hitting that share button, their messages were getting an unexpected tagline: “Sent from Firefox 🦊” with a direct link to the Mozilla app store listing.
The feature appeared quietly, with no fanfare or announcement from Mozilla.
https://piunikaweb.com/2025/11/26/sent-from-firefox-whatsapp-share-signature/
For now. Like many other features. Its the existence of it in the primary code that's the problem. They have a plugin system.