The effort, code-named "Electrolysis" (shortened to "e10s"), separated Firefox's operation into more than one CPU process. The practice lets the browser take advantage of multi-processor systems for better performance, and segregates the browser's user interface (UI) and content to keep Firefox from fully crashing when a website or web app fails.
Other browsers, including Apple's Safari and Google's Chrome, already support multiple processes, albeit differently. Safari relies on a single process for the rendering engine, then spawns a new process for each tab's content. Meanwhile, Chrome assigns a new rendering process to each new tab.
Mozilla will eventually get to a Safari-style model, but the open-source developer is moving there in measured steps. Today's Firefox 49 was the second, and is to be followed by November's Firefox 50, which will further expand the multi-process audience.
The e10s edition of Firefox 49 will be offered to those users who only run one or more of a small number of third-party browser add-ons that Mozilla has identified as compatible with the process separation. Firefox 50 e10s will reach even more users as Mozilla gives the green light to a larger group of add-ons.