Windows Pro version... try testing in Windows Sandbox (VM) to see if you have the same issue. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

The file in your temp directory that for some reason you're opening with Firefox ends with .htm not text or pdf.

Thank you jonzn4SUSE -

The files I send to the browser are not HTML files. They automatically open in the browser, one does not have to "Send To Browser" with them. They are normally .pdf files or files I send from my eMail client to the browser. But if Chrome can read them correctly as shown in the samples I attached to my original question, why can't Firefox?

I am not using an App. This is a Win 10 Pro desktop using a standard version of the Browsers Firefox and Chrome.

Using Sandbox will not be a useful exercise. This is strictly a UTF-8 character translation issue.

Here is what your screenshot is showing for the file name. see screenshot See also sample text file screenshot

Sorry jonzn4SUSE -

I misunderstood your message, but I answered it correctly i.e. The files I am sending to my browsers ARE .pdf and other text files.

The facts are that in Win 10 when one sends a file to a browser from a mail client, the mail client creates a temporary file in a TEMP folder that is opened by the browser and yes, it is an .htm file.

The semantics confused me jonzn4SUSE I was writing about what I SENT to the browser. You quite rightly point out that the browser opened file with a .htm extension.

OK. We now are on the same page and I once again ask the same questions: If Chrome and Edge are able to open the file without all the strange characters, why can't Firefox read the file correctly? As I pointed out in my earlier response to you, this is strictly a UTF-8 Character translation error.

Have I set a configuration incorrectly?

The steps to replicate would be helpful.

Sorry jonzn4SUSE -

That I cannot help you with because you do not use my mail client and probably do not use a mail client that allows you to send messages to your browser.

Thank you for trying to help, but I do not think your vector can do so.

I guess I need to run this past a Mozilla engineer of developer. It is related to the message viewing engine that Firefox employs to display a file and how it translates ASCII characters into extended ASCII - I am really not knowledgeable enough about this problem to explain it properly and understandably.

The closest I can come to a salient explanation is:

Message-File Viewer allows us to read and manage MIME-formatted e-mail messages files. It supports Latin, Cyrillic, Greek, Hebrew, Japanese, Chinese and many other character-set encodings in all 3 major standards - Unicode, ISO and Windows.

The Message-File Viewer in my mail client does not allow me us to read messages written in foreign alphabets in Windows and ISO flavors as it misinterprets UTF-8 characters and cannot take advantage of Message File Viewers that do support this capacity.

Hence, messages that need this support cannot be sent to Firefox as the way I have configured it (if the problem is indeed due to my configuration and not Firefox's message viewer) does not support this capacity either.

I hope that gives you some idea as to what this is about.

Perhaps there is a way you can direct me to deliver this message to a Firefox development team member or tech support person.

thank you so much for trying.

I would mark this as resolved and go open a bug to get a reply from Dev. https://bugzilla.mozilla.org/home

Note that this sometimes can happen if the website has pasted Unicode text to a page that is send by the server as an eight-bit Windows or Western encoding. If the sever sends an encoding then Firefox ignores an encoding set in the HTML page code.

Does it help if you try "View -> Repair Text Encoding"?

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?