Firefox itself is quite fast [1]; if you're having issues, there's a good chance that it's due to an add-on [2]. A (historically; I'm unsure if this is still the case) particularly egregious offender [3] is AdBlock Plus, but µBlock [4] does come highly recommended.
[2] https://support.mozilla.org/en-US/kb/firefox-uses-too-much-m...
[3] https://blog.mozilla.org/nnethercote/2014/05/14/adblock-plus...