In its latest release, Firefox will use up to four processes, with the benefits to users being better memory usage and performance.
But long-time users of Mozilla's browser may be disappointed to see that their browser is clinging to a single process -- and the guilty party are those add-ons you've probably been using for years.
To check if Firefox 54 is stuck on one process, you can check your nearest operating system process monitor, or check if the value attached to dom.ipc.processCount value in Firefox's about:config is set to 1.
If this turns out to be so, it's time to dig into Firefox's add-ons to see which extension is at fault.
Firefox provides little feedback on this front, but if you install the Add-on Compatibility Reporter extension, a wealth of information appears in Firefox's Extensions panel.
First up, in the top-right corner will be a label stating whether multiprocess is enabled or not.
Following that will be a list of all your extensions and a label saying whether or not they are compatible with Electrolysis.
What you do from this point is up to you: If you disable the incompatible extensions then multiprocess will be enabled, but you may have to bump up the dom.ipc.processCount value like I did to see it fully in action; or alternatively, you can push on in a single process world because you need a certain add-on, but keep in mind that come Firefox 57 in November, all extensions will have to be moved across to WebExtensions.