Mozilla plans to give Firefox users more control over the browser's performance options. In a recent announcement, the non-profit company stated that by giving the users more control over Firefox's ways of doing things, it will ensure that everyone can create the best possible browsing experience for themselves. At the moment, these new controls are still being worked on, so it might be a while until we actually see them in action, but the idea itself seems quite great.
As far as I understand, the new options will enable people to control things like enabling or disabling the animated transitions, using prefetching to load links ahead of time on the page that you're currently visiting, limiting how many processes the browser can run at a time, etc. This means that you won't only be able to make Firefox run smoother even if you have a lower-end PC, but that you will also be able to better manage the browser's data usage, which is great for those with capped connections. Additionally, there will also be an Optimize Firefox button that will help out those who don't want to get their hands dirty by manually fine-tuning the application. I for one, think that this move away from the one-size-fits-all approach is great; hopefully, Mozilla will find the right way to implement it.
As I said in the beginning, these new controls are still being developed so we might not see them in the near future. However, meanwhile, you might be interested in learning how to control Firefox using just your keyboard.