That pref is no longer supported in current Firefox versions. What reason do you have for disabling multi-process as it is not recommended as this can cause issues ? You can possibly disable Fission and set the number of content processes to 1. You can control Fission via <b>fission.autostart</b> on the <b>about:config</b> page, true to enable and false to turn it off. *about:config => **fission.autostart = false **dom.ipc.processCount You can open the <b>about:config</b> page via the location/address bar. On the warning page, you can click "Accept the Risk and Continue" to open about:config. *https://support.mozilla.org/en-US/kb/about-config-editor-firefox