|Firefox is a web browser. HTML files are web documents. You can open web documents with any web browser like Firefox, Internet Explorer, Chrome, Opera, Safari, etc.|
I don't understand your query, so please explain better. If you mean HTML documents on your computer now have a Firefox logo, it doesn't really matter because they can be opened using the Open With option in the context menu (right-click menu). You could also try opening your preferred web browser and locating the "Make program default web browser".
Apologies if I don't respond to your reply immediately. I don't check this site daily, but you're welcome to PM me as a reminder.