This is caused by using an autoconfig.cfg file in the Firefox installation folder to deploy/customize Firefox that is no longer compatible. *https://support.mozilla.org/en-US/kb/customizing-firefox-using-autoconfig Did you create this file yourself ? You can check the Browser Console for more detail. *https://firefox-source-docs.mozilla.org/devtools-user/browser_console/ Note that in Firefox 117 this can possibly be caused by having a Cu.import rule to import Services.jsm, but since you are on Windows 7 and thus use 115 ESR this wouldn't be the case. *[/questions/1419936] Firefox Developer edition error message "Failed tor read configuration file " on startup