Firefox preventing auto sleep

Firefox (78.0.2) is preventing my 2011 MacBook Pro (OS 10.13.6) from going to sleep. I can put it to sleep manually but no matter how few windows or tabs are open, it won’t sleep automatically. I’m not sure but I think this started after the most recent Firefox update.

Thanks in advance,
Don Frazer

So if you Quit Firefox, macOS will sleep on idle but if Firefox is running, even with no page but about:blank open, it won’t? It is supposed to be possible for a browser to prevent a computer from going to sleep but only in specific circumstances (e.g. playing video in full screen mode).

What if you disable all your browser extensions (go to Help > Restart with addons disabled) and have nothing but about:blank open, any change?

Have you shut down or restarted your Mac since Firefox last updated?

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During further investigations I noticed that the kernel_task process was taking up 30-50% of the CPU. Some Googling seemed to recommend that I reset the SMC so I did that (I was having a lot of fan activity also). I had to do a restart as part of that process so I can’t be sure what solved the sleep problem but it seems to be OK now.

Thanks for your recommendations.

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