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

https://developer.mozilla.org/en/Mozilla_Source_Code_Directory_Structure

For information on how to build Mozilla from the source code, see:

https://developer.mozilla.org/en/docs/Build_Documentation

To have your bug fix / feature added to Mozilla, you should create a patch and submit it to Bugzilla (https://bugzilla.mozilla.org). Instructions are at:

https://developer.mozilla.org/en/docs/Creating_a_patch
https://developer.mozilla.org/en/docs/Getting_your_patch_in_the_tree

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 in a mozilla.* Usenet group, or on IRC at irc.mozilla.org. [The Mozilla news groups are accessible on Google Groups, or news.mozilla.org with a NNTP reader.]

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.