Does this show when you are actively using Firefox, or when you have not even started Firefox yet?

When Firefox is running you can open the internal About Processes page by typing or pasting about:processes into the address bar and pressing Enter to load it. If you compare that list with the Windows 10 Task Manager's "Details" tab, they should have the same number of Firefox processes. If the Task Manager has additional processes, that requires further investigation.

If Firefox is running hidden in the background -- other than running briefly to check for updates -- that is suspicious and can be a sign of a malware infection.

It is quite normal to see more Firefox processes running.

• process for the main Firefox thread (user interface)
• one or more content processes
  you see more content processes when Fission is enabled
• process for the compositor thread
• process for extensions
• process for WebRender when this feature is enabled

There are several toolkit.aboutProcesses prefs on the about:config page to show more info. You can find the current multi-process state on the Troubleshooting Information page (about:support).

• "Help -> Troubleshooting Information"
  "Application Basics":<be>Multiprocess Windows and Fission Windows
  "Remote Processes" (Web Content)

You can check the about:memory and about:processes pages for info about all processes. See also the about:unloads page for info about the tabs.

• https://Firefox-source-docs.mozilla.org/browser/tabunloader/