For information on how to build Mozilla from the source code, see:
http://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 (http://bugzilla.mozilla.org). Instructions are at:
http://developer.mozilla.org/en/docs/Creating_a_patch
http://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 http://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.]
You can download nightly development builds from the the Mozilla FTP server.
Keep in mind that nightly builds, which are used by Mozilla developers for
testing, may be buggy. Firefox nightlies, for example, can be found at:
ftp://ftp.mozilla.org/pub/firefox/nightly/latest-trunk/