Firefox is not a operating system. You interface with that then open Firefox. Team Viewer works well https://www.teamviewer.com/en/

What are you using ?

Your Extensions could be a issue or your Security Software. Is the connections enabled in windows 10 to do so ?

What happens when the remote user tries to start Firefox? Is there a specific error message you can report?

Does the remote user have their own Windows profile on the machine, and their own Firefox profile under their User folder?