On 5/6/2019 at 4:36 PM, win32 said:Firefox 54 is the first multithreaded version
You can force "multiprocess mode" (aka "e10s") in FF 52 by creating the Boolean preference browser.tabs.remote.force-enable and setting it to true.
This also works in Serpent, BTW. Don't try this in New Moon, though; it will crash! It doesn't work in BNavigator either; setting the above preference makes it appear to work in about:support, but Windows Task Manager nevertheless shows only one task no matter how many tabs are open. 
Edit: Summarizing what was discovered below, this preference is required to enable e10s (multiprocess mode) in FF 52 on Windows XP. But with later Windows versions, or some other FF forks on XP (confirmed with Serpent 55), you can instead use the preferences @VistaLover recommends in the next post: toggle browser.tabs.remote.autostart from false to true, and if necessary (due to add-ons flagged as incompatible with e10s), toggle extensions.e10sBlocksEnabling from true to false.
This splits FF (or Serpent) into two processes. To allow more than two, set dom.ipc.processCount to a value greater than one (i.e., setting it to 2 allows 3 processes, etc.)
In Chrome, each tab is a separate process; but in FF the maximum number of processes is limited by that preference. If you open more tabs than that value, tabs will begin to share processes vs. creating new processes.