Give example site that gave you that error and let others save it and then open it to see if they get the same results.

Actually, most files involved are webpages that I saved and are receipts for orders I placed. So they are not available as a link I can post. Plus I don't want to post any personal financial info. But the odd thing is, several will open but several will not. They all show as identical file types and all show content when I go to properties. But, some just show white screen

Did you save those "pages" in MAFF or MHTML format using the (now) defunct MAFF or UnMHT extensions?

If so, unfortunately Firefox doesn't support those files types natively. Without those extensions being installed Firefox will try its best to display them, but certain aspects of the saved "page" may not be displayed properly or at all.

Personally, I have over 12 years of MAFF and MHTML files saved and I am keeping an older version of Firefox installed with those extensions so that I can view those files types if I would need to view them in the future.

Years ago when I started using MAFF and/or UnMHT I knew that they weren't officially recognized formats and the related Firefox extensions might always be needed to view them. IE does support MHTML, because they "proposed" that format to the W3C back in 1999, but the W3C standards organization has never accepted that "proposal" and made it a "standard". And probably why Mozilla never supported it in Firefox. IMO, W3C standards are read like a "bible" at Mozilla; if it ain't in "the good book" it won't be supported.

Do the files have actual content if you open them in a text editor or in another browser?

Do the files have the correct file extension (.html)?

Do you see anything if you give them a .txt file extension?