First install an SSD if you haven’t already. Next install ublock origin in Firefox ESR and tighten down the security settings to max plus turn off all telemetry, studies and other “features.” Don’t use a Mozilla account as that adds overhead.
It still will be slow but it should be usable with a few tabs. Do not try to do video playback as the old GPU doesn’t support modern video formats so the CPU ends up decoding it all.