Yes, Mozilla Firefox is an open source software. It was initially developed by the Mozilla foundation and also by the Mozilla Corporation. It supports most systems including windows, Linux, macOS, Solaris, BSD and illumos. It is also available for android.
Thanks to it being an open source, different communities can contribute to the software, making it an even better interface. This may also be the reason why Firefox has such an extraordinary privacy all and user interface. It builds on a variety of input and is able to update itself with the latest innovative technologies giving users an exceptional experience.