It’s Open-Source
Firefox is an open-source platform, which means that anyone can tweak or contribute to its code. Unless you’re an open-source advocate, that may not mean much to you. But it means that the Firefox app itself is much more transparent than Chrome or Safari, and it’s also more customizable.
There are plenty of themes that let you change the look and feel of the browser itself. Many third-party extensions can also add new capabilities and features to Firefox. And if you have some technical skills, there’s actually a community of people dedicated to tweaking code to edit Firefox’s appearance and functionality.