An explanation of the Firefox Source Code Directory Structure and links to project pages with documentation can be found at:

https://firefox-source-docs.mozilla.org/contributing/directory_structure.html

For information on how to build Firefox from the source code and create the patch see:

https://firefox-source-docs.mozilla.org/contributing/contribution_quickref.html

If you have a question about developing Firefox, and can't find the solution on https://firefox-source-docs.mozilla.org/, you can try asking your question on Matrix at chat.mozilla.org in Introduction (https://chat.mozilla.org/#/room/#introduction:mozilla.org) channel.

Nightly development builds can be downloaded from:

https://archive.mozilla.org/pub/firefox/nightly/latest-mozilla-central/
        - or -
https://www.mozilla.org/firefox/channel/desktop/#nightly

Keep in mind that nightly builds, which are used by Firefox developers for testing, may be buggy.