[yitosda]

Try to create a new profile to compare nondestructively: "firefox --ProfileManager"

I can confirm both that newer firefox is notably more responsive than old, and that I (or an extension, though I don't have many) managed to get a ff profile into a state where it had inexplicably bad performance, esp. unpredictable long freezes.

[cpeterso]

You can also create and manage Firefox profiles using the about:profiles page.