content: Firefox is pretty darn solid. Too many add-ons can slow it down though. Ghostery was a notorious offender because it conflicted with other security add-ons.  Not to say you can't make it snappier still. Follow ArchWiki's performance tweaks and you can halve the memory footprint, among other things.  Most of the tweaks are applicable in all operating systems but since it is Arch Linux's wiki a certain level of competence is expected from the user.  In the same vein you can follow https://www.privacytools.io/ for privacy related tweaks.