In the past Mozilla has claimed that Firefox is used quite a bit on older machines with less than optimal specs, which is why some users are complaining about the browser’s performance when it really isn’t Mozilla’s fault. However to address that, Mozilla is making some changes, one of which is an “optimize Firefox” button.
When pressed, this button basically kills/disables any extensions that you might be using, thus making the browser potentially run a lot smoother. The extensions will still be there and users can re-enable them after. Another feature Mozilla is working on is in the form of content processes, where in future builds it will support three or more content processes versus the two (core and content) right now.
There will also be options for users to disable certain UI features, such as animations for tabs, menus, and so on. The idea is that if you have a computer that doesn’t have much RAM, disabling these features could help speed things up. These features are still in the works which means that you might not be able to expect them to be released anytime soon, but they could be worth looking out for in the future.