Building Firefox

If you want to start building Firefox itself then the best place to start is Getting Set Up To Work On The Firefox Codebase — Firefox Source Docs documentation which covers Windows, Linux and macOS and is the official starting point.

The instructions are reasonably easy to follow, however you do need quite a bit of disk space to download everything, it is also worth following the recommended approach and using Visual Studio Code as an IDE, installing all of the recommended extensions.