Major performance improvements were made in Firefox version 48 

http://arstechnica.com/information-technology/2016/08/firefox-48-ships-bringing-rust-mainstream-and-multiprocess-for-some/

http://www.pcworld.com/article/3103497/software/extensive-firefox-revamp-safeguards-against-freezes-crashes-and-lag.html

After seven years of development, version 48 is at last enabling a multiprocess feature comparable to what Internet Explorer and Google Chrome have offered as stable features since 2009. By running their rendering engines in a separate process from the browser shell, IE and Chrome are more stable (a webpage crash does not take down the entire browser) and more secure (those separate processes can run with limited user privileges). In order to bring the same multiprocess capability to Firefox, Mozilla started the Electrolysis project in 2009. But the organization has taken substantially longer than Microsoft, Google, and Apple to ship this feature.

As much as Firefox lags in this area, it’s cutting edge in another. Mozilla’s Rust language is designed to give the same level of performance and control as C++ but without C++’s susceptibility to security flaws. The company has developed Rust code to replace the C++ code that currently handles complex media formats. This replacement code is now shipping in the stable version of Firefox 48.