Hi, could you switch to the "Details" tab in the Windows 10 Task Manager -- it lists the Process ID number.

Then, leaving that open, in Firefox, could you open the built-in Process Manager (Shift+Esc, or type/paste about:processes in the address bar and press Enter to open it).

These two lists should match, process for process, and you can see which processes are for which websites versus which are for other aspects of the browser. Maybe particular processes are using more memory than expected?

If the Windows Task Manager lists firefox.exe processes that are NOT listed on the about:processes page, then further investigation is needed because this sometimes indicates malware doing web stuff in an invisible window.

Thank you for your advice. I now know that I must have another problem. The detail page shows 14 entries for Firefox. The about:processes show only 2. There must be another problem. I just ran a scan yesterday.

That's strange. Usually there are more than 2.

The Windows Task Manager allows adding a Command Line column. The typical command lines are the one you use the start Firefox, like

"C:\Program Files\Mozilla Firefox\firefox.exe"

and then a bunch of related processes that look like

"C:\Program Files\Mozilla Firefox\firefox.exe" -contentproc --channel=12345

followed by -childID or -parentBuildID

What would be suspicious is something like

"C:\Program Files\Mozilla Firefox\firefox.exe" -headless -P someprofile "https://example.com

in other words, a separate instance of Firefox running invisibly on some specified site.