Mozilla follows the same model is Microsoft Code with Firefox that the source code is FOSS but it's bineries are not.
Firefox, the executable application, is not open source and is considered "source available" because the Terms of Service contains exclusions that are against the tenants of freedoms that must exist in open source software.
Firefox, the source code, is still licensed via Mozilla Public License and is open source. If you compile it yourself, you must call it something other than Firefox.