Rated 2 out of 5 stars

Firebug already provides this information. You can right click the item on Html view and find "Copy Xpath."

This review is for a previous version of the add-on (0.9.2). 

FireXPath has other functionalities that Firebug doesn't have

True, however with Firebug you won't be able to visualize the result of the evaluation of an XPath expression. You can still evaluate XPath using the $x function in the command line but it won't be as convenient as using FireXPath. If the only thing you want is to copy the absolute XPath from a single node of an HTML document then you don't need FireXPath.