XULPlanet (http://www.xulplanet.com/): An excellent resource packed with Mozilla-related guides, tutorials, and examples. The site has several reference sections covering everything from XUL to XPCOM components.
Mozilla.org (http://www.mozilla.org): Has a lot of useful information for developers. Most of it is linked from the documentation page at http://www.mozilla.org/docs/, but there are many additional resources scattered around the site. You can do a site search to try to find the needed information.
MozillaZine.org knowledge base (http://kb.mozillazine.org): A usercontributed wiki with many useful articles, guides, and links to additional resources. The Development section has a lot of information on extension programming.
MozillaZine forums (http://forums.mozillazine.org/): Post your questions and comments here. The site has a Mozilla development section with a forum dedicated to extensions.
Netscape.public.mozilla newsgroups (http://www.mozilla.org/community/developer-forums.html): You can search the newsgroups for the wanted information or post your Mozilla development-related questions. The Mozilla.org site has a list of the available newsgroups and their topics.
Internet Relay Chat (IRC) (irc://irc.mozilla.org/): There are several IRC channels you can visit to chat with your fellow Mozilla developers in real time. Several developer channels, including #developers, #mozilla, and others, can be found on the Mozilla.org IRC server.
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