I'm still using the Fox. It's better now. I found some tweaks. I browsed quite a few guides, and found out that quite a few old options no longer apply in the new version... but some still do! And they help, a little, so here's a little list of those for those of you who might need them.
For starters, type in
about:config in your address bar, and then experiment (at your own risk) with setting the following:
browser.download.animateNotifications to False
security.dialog_enable_delay to 0
network.dns.disableIPv6 to True
browser.tabs.animate to False
browser.display.show_image_placeholders to False
browser.sessionhistory.max_total_viewers to 0
For finishers, type in
about:memory in your address bar, and 'Minimize memory usage'. This should minimize memory usage.
A few other things I've been doing a long time now that speed things up: keep any extensions you don't need disabled by default, clear your cache and history every once in a while, don't let the browser add-on list get bloated, and if all else fails:
about:support. Reset the browser.
Resetting the browser will reset some things, of course, so use that as a last resort if you have settings and history you'd like to save. It's been a while since I last did that. For now, these tweaks seem to be working pretty well.