Installed this yesterday (from Linux mint), all seemed to go well, after the install "restart firefox" which I clicked, then things got really slow, running top

Installed this yesterday (from Linux mint), all seemed to go well, after the install "restart firefox" which I clicked, then things got really slow, running top showed a load of 120 (on a 4 CPU machine!), locked the machine pretty hard, 10 minutes later a "firefox has crashed" dialogue, which I clicked only to find another underneath, then another, then another. Around 20 in all. Not your finest moment Firefox...