Firefox Project Fission is an ongoing development effort that is supposed to fundamentally improve the security and privacy of the web browser. It will finally introduce site isolation to Firefox which sandboxes web pages, completely isolating them from each other. Websites are already not supposed to access each other's data but security bugs do happen and attackers can exploit these.
A quick example: In the current state, this entire page runs in a single process and an undisclosed bug in Firefox could theoretically allow example.com to access data from the embedded social media sites. With site isolation, all three websites would run in separate processes and each would reject any requests from other, potentially malicious processes. Project Fission is not finished yet but you can try it out by opening the about:config page in Firefox and enabling the values "fission.autostart" and "gfx.webrender.all".