Despite their popularity, both Firefox and Chrome have reputations as resource hogs that chew up big chunks of your RAM. In the case of Firefox, some of this is unfair -- parent Mozilla says part of the problem is that many users are running older machines without a lot of memory in the first place. To help those folks, the developers are working on a new feature called "performance" that will let you fine-tune the browser if you're running a PC that's less than state-of-the-art.
Firefox plays better than most browsers with third-party add-ons, but as the developers have pointed out before, some of those can slow your machine significantly. As such, they're working on an "optimize Firefox" button that kills all extensions after you restart the browser (below). That alone could make things work a lot better, and "you can always enable them again later," the developers say.
Another potential new feature is "content processes." Right now, Firefox only supports two processes (for the core and content), but starting with version 55, it will support three or more, putting it on par with Chrome, Safari and others. With the performance settings, the developers will give you more granular control of those, so you can trade off speed for memory usage. "More content processes will make Firefox more responsive when using multiple tabs, but will also consume more memory," according to the proposed UX.
Users of slow PCs will also get to disable UI animations for tabs, menus and other things, and use prefetching, which "can improve performance when on a slow connection." If you have a machine with a new CPU and plenty of RAM, you can probably just leave all the settings enabled.
Work has just begun on this feature, so don't expect to see it in a release or even nightly build for a while. Firefox is on version 52 right now, with version 53 set to come on April 18th, and as mentioned, support for more than two processes won't come until Firefox 55. Consequently, the performance tab probably won't be released to the public until somewhere near the end of the year.