FF 54's "big change" is full-implementation of up-to-4 multiple FF processes ("electrolysis", or "e10s"), to make FF run faster, use less memory, and be less crash-prone.
However, it turns out that certain FF extensions, such as WOT, are not compatible with electrolysis, and must be disabled if you wish to take advantage of electrolysis.
To find out whether or not you ARE using electrolysis, and if not, which add-ons are the culprit(s), there's a FF extension called "Add-on Compatibility Reporter" https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/ . When installed, go to Tools/Add-ons/Extensions. Toward the upper right, it will advise you as to whether or not FF's Multiprocess is enabled. It will also clearly show, next to each extension, an informational note that it's compatible with multiprocess, or a yellow triangular warning that it's not compatible with multiprocess. If you decide you can live without the incompatible add-ons, then you can disable them, and restart FF to see if multiprocessing is now running. For example, in task manager, I now see 3 FF processes running (after disabling WOT).
Reference: http://www.zdnet.com/article/why-your-firefox-is-not-using-multiple-processes/