Instructions:

To build the plugin, just run 'ant' to create build/xpathchecker.xpi.

To run the unit tests, you will need to download and install jsunit (not
included) and open 'test/test.html' from within jsunit.

The xpathchecker.ipr and xpathchecker.iml are configuration files for
IntelliJ IDEA.

The build.xml file contains other targets for automatically installing
the plugin in firefox, but they will need modification for anyone else
to use them.

Plans:

I don't plan on making any changes other than to make sure that XPath
Checker works with new versions of Firefox.  However, I'll be happy to
review any patches that you want to send my way.  (It may take a while.)

Ideas:

I've entered a few ideas into the Issue tracker:

http://code.google.com/p/xpathchecker/issues/list?can=2&q=type:Feature

None of them will happen unless someone volunteers to do it.