Earlier this week Mozilla released Firefox 58.0. The new version makes significant improvements its graphics engine and Javascript handling, which should translate into faster page rendering, especially on sites that use a lot of Javascript. Mozilla says we can expect further performance improvements in Firefox in the coming weeks.
Note that Firefox 58.0 user profiles are not compatible with earlier versions of Firefox, so if you don’t like 58.0 and decide to downgrade, you’ll have to create a new profile.