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 fission.autostart on the about:config page, true to enable and false to turn it off.

• about:config =>
  • fission.autostart = false
  • dom.ipc.processCount

You can open the about:config 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

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