Well, I do open a lot of tabs & a lot of FF instances (One pro, one perso, one for personal business, one for testings…) and my 10 years old quad core I3 with 12Go DDR3 can cope with it.
Webrender :
You can also set browser.tabs.unloadOnLowMemory to true
On your FF you can add localcdn and auto tab discard
For swappiness, the goal is to maximize RAM usage, thus:
echo vm.swappiness=5 | sudo tee -a /etc/sysctl.d/99-swappiness.conf
echo vm.vfs_cache_pressure=50 | sudo tee -a /etc/sysctl.d/99-swappiness.conf
sudo sysctl -p /etc/sysctl.d/99-swappiness.conf
Auto-cpufreq is in community repositories.
https://forum.manjaro.org/t/favorite-lesser-known-apps/5481/71?u=nam1962
You can add ananicy
https://forum.manjaro.org/t/favorite-lesser-known-apps/5481/108?u=nam1962
After all this, tell us if your PC has less freezes 