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CPU spikes - temporary 100% CPU usage, even with add-ons disabled

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I've been getting huge CPU spikes in the past month(s). I don't remember exactly when it started, but at least 3 weeks ago. Firefox completely freezes, which means I'm not even able to check the about:performance tab to see what's causing the spikes. The first uploaded image shows a frozen tab, all my tabs look like this.

This also happens in safe mode, with all add-ons disabled and a default configuration. I haven't tried completely reinstalling FF yet, as I was hoping an update would fix this. The second uploaded image shows the output from the top command during such a spike.

What can I do to better diagnose the problem? Are there external tools I can use to find the source of the problem?

I'm running FF 66.0.3 on an up-to-date Ubuntu 18.04.02, on a Lenovo T480s with an i7-8550u and 16G of memory. Can I provide any other useful information?


Attached screenshots

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