I'm running the latest version of FireFox. This has warned me about entering two or three potentially malicious sites, but I get multiple occurences daily of pages virtually freezing and a warning pop-up that the plugin is slowing FF down. I've therefore disabled it for now. When I next remember, I'll give it another try to see if the issue (if there is one) has been addressed.

Edit: VERY impressed by the speed of Malwarebytes' response! Have added a diary reminder to turn it back on in a week's time to see if the fix has been implemented. In the meantime, I'll try very hard not to visit sites I shouldn't. :-)