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.