Unfortunately, that combination of symptoms can indicate there is malware operating its own instance of Firefox (for example, doing Bitcoin mining). In the Windows 10 Task Manager, if you click over to the Details tab, you can add a new column for "Command Line" which will show more information about where each program is running from. To add a new column, right-click any current column heading (like Name, CPU, etc.) and click Select columns. Then add Command Line. Is the high-CPU instance running from a different folder than the regular instances? The location may reveal more about the culprit. Also, what parameters does it start up with? Sometimes searching those will discover other pages on the web where people have identified the culprit. And of course, meanwhile, you can run malware cleaners like the ones listed in the support article: [[Troubleshoot Firefox issues caused by malware]]