I had been working on this for a while but i was able to solve this for anyone who faces the same issue.
Basically if you import the office.interop dll version 126.96.36.199 it will automatically update to the newest version. The newest version will generate excel files in the .xlsx format (even though we never specified that). SOMEHOW Internet Explorer is able to parse a xlsx file as xls and you are fine, but Mozilla will not.
Basically set the Application.DefaultSaveFormat = 56 and you will be good for making .xls files.