Updated Malwarebytes Free to new version - and it went into free trial period for Malwarebytes Premium.
Soon afterwards Firefox began to fail - it would freeze, and using task manager to kill it would remove all but one process that became unkillable without rebooting machine. That process would also prevent Firefox from restarting.
Switched to Opera - same result
Switched to Chrome - same result
Switched to Internet Explorer - same result.
Resolved problem by ending the Malwarebytes trial and reverting to the Malwarebytes free without Web Protection.