Do you think it means that because Firefox is still running, it can't do the update? After you quit Firefox using either:

• "3-bar" menu button > Exit
• (Menu bar or Alt+F) File > Exit

you can check the Windows Task Manager (Ctrl+Shift+Esc) to make sure Firefox shuts down within 60 seconds. If it lingers after that, you can End Task or End Process (however it worked in your version of Windows Task Manager) to make sure firefox.exe is no longer running.

Any difference?

Oh, also, do you trust Microsoft, or do you want to back up your Firefox data? See: Back up and restore information in Firefox profiles