A suggestion that I believe would greatly benefit Firefox would be the adoption of the technologies of the moment: Artificial Intelligence (A.I.) and Deep Learning. Widely used by AMD and NVIDIA in their high-end GPUs for better graphics/performance in games and other software. Both are focusing more on making intelligent software improvements using AI.
1. One way to use artificial intelligence to improve Firefox's performance would be to apply machine learning algorithms to optimize browser settings, according to the user's profile and preferences.
For example: Firefox could learn which sites a user visits most and preload them in the background to reduce waiting times. Or, Firefox could automatically adjust the “Performance” option in the settings, to balance memory and processor usage, according to user demand.
2. Another way to use A.I. to speed up the browser would be to implement artificial intelligence systems that could assist the browsing engine.
For example: Firefox could use artificial neural networks to analyze the source code of web pages, and identify possible errors or inefficiencies that could affect their loading or rendering. Or, you could use genetic algorithms to evolve your own source code, seeking faster and more robust solutions to the problems encountered.