Hi, check the built-in Task Manager (Shift+Esc or about:processes) to see what is using all those processes. There will always be a few for Firefox itself, GPU, and extensions, even with only 1 tab open. Are there unexpected sites listed here?

The Windows Task Manager's "Details" tab (look for that in the column of buttons on the left) shows the Process ID that should appear on the built-in Task Manager list. . If there is a discrepancy between the two, this could indicate a hidden session running, either to check for updates or, more worryingly, to run malware in the background.