Could use more detail about how to use an existing Firefox extension

[davidmichaelkarr]

The section about using a Firefox extension is helpful, but it could use a little more detail.

For instance, this specifies the path to a firefox extension. Where would I have gotten this file from? There are ".xpi" files in my ~/.mozilla tree, including the one I want to use. Can I just copy that ".xpi" file to my local project tree, or what? It would be helpful if this section covered that.

It seems likely to me that one of the most common extensions someone would want to use for this is Firebug. An example using that exact extension would help.