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Firefox Got Faster for Real Users in 2023
(hacks.mozilla.org)
This is a most excellent place for technology news and articles.
Then something is fishy with your install. Maybe something to do with the DNS or the Firefox version you're using, or maybe just Debian.
DNS is not to blame here since Chrome and other applications work perfectly fine. Firefox in question is Developer Edition, so it's bleeding edge new. And it's not Debian because everything works fine until I sync my profile, then things slow down.