Resources, tutorials, samples... on developing extensions

Web:

http://www.mozilla.org/projects/firefox/extensions/

How to write Firefox extensions:
http://extensions.roachfiend.com/howto.php

Writing Firefox/Thunderbird Extensions
http://www.bengoodger.com/software/mb/extensions/howto.html

From "the law of averages >> Writing Mozilla Extensions":
http://www.yergler.net/blog/archives/2004/05/20/writing-mozilla-extensions
http://devedge.netscape.com/viewsource/2002/toolbar/
GUID Generator: http://extensions.roachfiend.com/cgi-bin/guid.pl
Firefox Extension RDF Generator: http://ted.mielczarek.org

Books:

"Creating Applications with Mozilla" by David Boswell, Brian King, Ian Oeschger, Pete Collins & Eric Murphy

Miscellaneous:

XUL Tutorial:
http://www.xulplanet.com/tutorials/xultu/

XUL Reference:
http://www.xulplanet.com/references/