A few years ago, before Firefox switched to the new rapid release cycle, Mozilla started working on re-architecturing the browser to make use of the multiple cores that most processors have today.
The idea was to improve performance and security by relying on parallelism.
Chrome has a multi-process architecture, and tabs, plugins, and several internal processes are separated from each other. But Google started from scratch with a multi-process layout in mind from the get-go.
Refactoring Firefox code to do the same proved too much of a hassle to be worth it and, after a couple of years of work, the project, dubbed Electrolysis, was put on hold.
But things seem to be moving again. Some members of the team have started working and submitting patches related to Electrolysis-related bugs again. Whether this is a sustained effort and Mozilla is making this a priority or whether it's just a few people making sporadic contributions remains to be seen.