I've had to quit using Cookie 126.96.36.199.8, as it is spiking CPU usage to 140% or so.
MacBook cycles from 0% to 140%, iMac remains steady at around 130%.
"App busy" spinning graphic appears in Cookie window.
But it's not frozen, as I can still delete tracking cookies in this state.
El Capitan 10.11.4, Cookie beta 188.8.131.52.8, running current Firefox and Safari