Today, Mozilla has added the Firefox profile path to about:support page in Firefox 58.
Profile Folder line under ‘Application Basics’ shows the Profile path in addition to the open folder button. This improvement makes user to know Firefox profile path more easily from about:support page without need to open the profile folder.
FYI, till now Firefox offers a button to open profile folder in the about:support or troubleshooting information page and Firefox’s about:profiles page also shows browser’s profile path.