Try XPath v1.3.5
This add-on displays the result of evaluating XPath expression or CSS selector.
This add-on requires <all_urls> permission in order to execute a script in a iframe in Firefox version 58.0.
Usage:
1. Open the web page you want to apply XPath to.
2. Click the add-on icon in the browser toolbar.
3. Enter the XPath expression in the expression input field.
4. Press the enter key or click the execute button.
The resulted elements will be surrounded by the dashed line and the results will be shown in the popup.
Features:
Specifying the context node.
Specifying the behavior of the namespaceResolver function.
Highlighting the found elements by the user-defined css.