Links:
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http://www.alistapart.com/ - all around excellent CSS resource
http://www.quirksmode.org/ - JS and CSS tips and documentation on browser incompatibilities
http://css.maxdesign.com.au/ - Tutorials on lists, floats, and selectors
http://www.w3c.com/ - World Wide Web Consortium (Documented standards for all that is CSS)

Firefox Extensions:
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http://www.mozilla.com/firefox/ - Firefox
http://chrispederick.com/work/webdeveloper/ - Web Developer extension; lots of goodies (Must have)
http://ietab.mozdev.org/ - IETab; View Internet Explorer rendered pages in Firefox
http://firefox.exxile.net/ - All-In-One Sidebar; Puts a lot of FF content and makes it accessible via sidebar