A little while ago, I created a Firefox Add-On.  My purpose was merely to familiarize myself with add-on/extension development.  The add-on (extension) I developed is called ReferenceThis and can be downloaded here:
ReferenceThis is a simple extension that provides a right-click option that displays the definition of the highlighted word or phrase (from Google define:). The definition will be displayed in whichever language is selected in the Preferences dialog box. All languages supported by Google define: are available for selection. ReferenceThis will also search a variety of reference sites (Wikipedia, NY Times Reference, and for the word or phrase. Additionally, the Preferences dialog box allows a custom entry so any site can be added. The four default sites can also be toggled on or off.
Development of the add-on was relatively straightforward.  A search on “firefox extension development” yielded all of the information I needed.  Here are a few of the more helpful articles: