Multiprocess Firefox, also known as "Electrolysis" (often shorted as "e10s"), offers a major improvement to your browsing experience by separating Web content and Firefox UI processes. This means when a web page is consuming a large part of your computer’s processing power, your tabs, buttons and menus won’t lock up.
The initial roll-out is happening in Firefox 48 on August 2, 2016. To learn more, head over to the Electrolysis wiki page or MDN.
To learn more about extension compatibility in multiprocess Firefox, click here.