What are the minimum programming requirements to create a Firefox plugin?
You need to learn XUL for the UI and Javascript for the programming.
more infos here:
https://developer.mozilla.org/en/Building_an_Extension
Mainly is XML/XUL, javascript and css. My favorite extension is Firebug though.
https://developer.mozilla.org/en/Building_an_Extension
http://lifehacker.com/software/programming/how-to-build-a-firefox-extension-264490.php
http://www.rietta.com/firefox/Tutorial/overview.html
Google is your friend
Firefox plugins (such as things like Flash player) are binary components you develop in NPAPI the cross-browser API for plugins.
Check out Mozilla Plugin Developer Center
You might also look into Firefox Extensions sometimes Firefox plugins are shipped as extensions. Firefox extensions modify or enhance the functionality of the browser itself. Javascript is primarily the language you'll be writing a Firefox plugin.
Check out the Mozilla Extensions Developer Center
Some Firefox plugins that come in handy when developing Firefox plugins are the Venkman Javascript Debugger and Firebug Javascript debugger
If you are into an organized style of reading, and prefer an good ol' book, start here. http://www.amazon.com/Programming-Firefox-Building-Internet-Applications/dp/0596102437 It will get you started on the basics on XUL, which is in turn used to build the addons. If you have a touch in javascript, it will definitely help you.
Else, Start here. https://developer.mozilla.org/En You can start learning what all you will need to start developing firefox addons, or even firefox itself.
If you are fairly proficient in Javascript and DOM, then, just jump here. https://developer.mozilla.org/En/Developing_add-ons. The only problem might be, they are as not comprehensive for a newbie programmer. If you have some professional background in programming, and can pick up a language from sample codes, then start here.
But I would definitely suggest a well organized book, like I mentioned above. There are very few XUL book in the market, this was my first google result, so just linked it here.
Search for XUL on http://www.amazon.com/.Choose one, atleast a book published after 2006-07.
[edit] Just found out this good article http://kb.mozillazine.org/Getting_started_with_extension_development
There seems to be a huge misunderstanding here; Are you trying to create a plugin or an extension? As Dougnukem stated, plugins have nothing to do with XUL or extensions. They are shared libraries (.dll, .so, or .dylib for win, linux, mac) that provide functionality that the browser isn't capable of alone.
If you're just getting started, I'd recommend looking at the FireBreath plugin framework:
http://firebreath.googlecode.com
and reading through some documentation. A decent place to start is: http://colonelpanic.net/2009/03/building-a-firefox-plugin-part-one/
Also, you still haven't selected an answer for this question; is that because you haven't found an answer yet, or because you've forgotten?