Because Firefox is a browser, not a family of products. Well, now it is, but they already had a brand for their family of products: Mozilla.
https://www.mozilla.org/en-US/firefox/
> Meet our family of products - Browsers
- Monitor
- Send
- Lockwise
So, for many years now, "Firefox" has been morphing into a brand that encompasses many online tools beyond the browser that are all intended to be tied together by Mozilla's mission & manifesto.