An explanation of the Mozilla Source Code Directory Structure and links to
project pages with documentation can be found at:
https://firefox-source-docs.mozilla.org/tools/docs/contribute/mozilla_source_code_directory_structure.html
For information on how to build Mozilla from the source code and create the patch see:
https://firefox-source-docs.mozilla.org/tools/docs/contribute/how_to_contribute_firefox.html
If you have a question about developing Mozilla, and can't find the solution
on https://developer.mozilla.org, you can try asking your question on IRC at irc.mozilla.org in #introduction 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 Mozilla developers for
testing, may be buggy.