Firefox 54 is significant in that the work of the Electrolysis project is now enabled for everyone: this is the multi-process support in Firefox designed for a more efficient web browser particularly around utilizing multiple tabs and/or content heavy sites. Firefox 54 will make use of up to four processes for dealing with web content.

Firefox 54 also improves bookmark sync performance, enables 5.1 surround sound by default for all Firefox desktop builds, support for VP9 video within encrypted Clearkey/Widevine video streams, security fixes, and other web developer enhancements.

https://www.nextinpact.com/news/104553-firefox-54-generalise-multiprocessus-a-tous-utilisateurs.htm
https://linuxfr.org/news/printemps-2017-de-mozilla-firefox-52-a-54-et-thunderbird-52