A quick look at Mozilla's product page with 12 products of which five are different browsers, explains why the decline has happened: https://support.mozilla.org/en-US/products/ That's a lot of products to support. Add to that this list of 27 discontinued products: https://killedbymozilla.com/ And you can understand why the struggle is real for firefox. It should be the best, bar none at this point.
All the browsers are probably counted as Firefox. A better guidance would be to count based on engines (Chrome, Edge, and Safari are all variations on WebKit) and have a device preventing an engine monopoly.
Firefox for iOS uses WebKit as the rendering engine, and the others are significantly divergent from Firefox proper and take significant development resources away from the core product, so I disagree with counting the products as a single browser.