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Mozilla plans to remove configuration options in upcoming versions of the Firefox web browser that allow users to disable the browser's multi-process architecture.
Mozilla plans to remove two Firefox preferences from any version of the web browser going forward. The change is planed for Firefox 68.
  • browser.tabs.remote.force-enable -- Enforces the use of multi-process in Firefox if it is not enabled by default, e.g. because of accessibility.
  • browser.tabs.remote.force-disable -- Disables multi-process in the Firefox web browser.
You probably wonder about browser.tabs.remote.autostart, the preference that enables or disables the multi-process architecture in Firefox (opposed to enforcing a value like the two other preferences do).

Mozilla will restrict the preference to true in home builds. While it is still in Firefox, setting it to false in about:config won't have the desired effect anymore once the change lands. The status of the preference is always true in home builds of Firefox regardless of the user chosen value.

In other words: going forward, Firefox users can't disable multi-process anymore in the browser. It is unclear in how many Firefox installations multi-process is disabled. Some users do so to improve memory usage or slowness of the browser.