It happens and i think it is normal as of now my pc is running 15 in my background on task manager,and it always does this.If your memory is not going exceptionally high like above 60 you should not worry.If you try closing them from last some might close without crashing but that won't help much.

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/