Mozilla has removed Firefox Health Report (about:healthreport page) in favor of telemetry.
Firefox Health Report made available with Firefox 22, which gives insights and performance information about your version of Firefox, has been removed by Mozilla in Firefox 59.
In current Firefox 57 Quantum, you can able to open FHR by selecting the option in the Help menu, which loads about:healthreport page and displays information about Firefox browser.
In Firefox version 59, Firefox team has removed access point to Firefox Health Report in the help menu and the feature also, as a result, you’ll now see invalid URL or address not found page when you visit about:healthreport.