Firefox has a performance enhancement function called Electrolysis that can use multiprocess windows. However, the Throttle Firefox extension blocks Firefox from using this capability, as do many other legacy extensions (see https://wiki.mozilla.org/Electrolysis).
Furthermore, the technology that Firefox uses for add-ons will be deprecated starting with Firefox 57, scheduled to be released in NOVEMBER 2017 (https://support.mozilla.org/en-US/kb/firefox-add-technology-modernizing).
When will Throttle transition their Firefox add-on to the WebExtension API, which is also used by Chrome & Opera and is compatible with multiprocess windows? (https://developer.mozilla.org/en-US/Add-ons/WebExtensions)