Installation can't be more straightforward. From the menu Tools>>Web Developer>>Get More Tools, go to the web page where you can find the FireBug add-on. Install it like you would with any other add-on. You can start it by pressing the F12 button on your keyboard. Then install the FirePath add-on from the same page.
It extends the FireBug add-on allowing real-time monitoring
To use FirePath, open the FireBug add-on, and navigate to the FirePath tab. From that tab, you can view and edit any web element. Just locate the XPath edit field, which points to the desired web element, and you can inspect and then edit your custom XPath, which can then be verified using the highlight button.
FirePath add-on makes inspecting, editing, and testing the XPath straightforward. You can copy the XPath of any selected web elements quickly and reuse the values in other projects.
FirePath add-on will let you extend the capabilities of your FireBug add-on by giving you the options to change, view, and make the XPath and CSS selector locations.
You need FireBug add-on prior to installing the FirePath add-on.