The add-on available at addons.mozilla.org is compatible

• https://addons.mozilla.org/en-US/firefox/addon/xpath-checker/

Error as same as previous .

PFA Screen Shot

Oh, I see

Most of the times after the release of a new Firefox version, add-ons may become incompatible. It's up to the developer to address these issues and update it accordingly. You can contact the developer from their add-ons page.

https://addons.mozilla.org/en-US/firefox/addon/xpath-checker/versions/ Version 0.4.4 Released July 25, 2010 6.1 KB Works with Firefox 3.0 and later

So this Extension rather has not been updated for almost four years now.