Same problem, probably for 2 - 3 months now. Only OS restart fixes it.
Bug 1615412 Comment 3 Edit History
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Same problem, probably for 2 - 3 months now. Only OS restart fixes it. No Firefox's processes are otherwise running. Re Morgan's screenshot: If toolbox fails to connect, in most cases it will have a full page error dump. Only very rarely I get Morgan's split error toolbox state and recently it only shows up for a moment and then recovers.