It might have to do with the amount of RAM on the computer? On a computer with 4GB of RAM, I've found FF crawls up to around 2GB; on a computer with 8, I've found it crawls up to around 4GB. Similar script blocking and host blocking setups on both, most recent versions of FF (though different versions of MacOS).

Similar horrendous effect on the performance of everything else on the computer til I restart the damn browser.