That's strange!!

How about a link to the inbox folder -- does this work?

https://outlook.live.com/mail/inbox

Also, your Firefox identified as either Beta or Developer Edition (Firefox 69). Those versions do get frequent changes, so keep an eye out for other reports of a new issue.

Thanks but no help. Did you see the message I get on the screenshot I attached to the post. It makes no sense. I wish firefox themselves could advise why I am getting that.

There's not a good explanation for why a web page from live.com should come with headers indicating it is a binary file (application/octet-stream) instead of HTML.

Have you already ruled out extensions as a potential culprit? You could test in Firefox's Safe Mode. In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help button > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Just tried safe opening of Firefox but still no joy with Outlook. As I said, Firefox is opening all other files.

I would expect more reports if it were caused by popular security software like Avast, AVG, Bitdefender, ESET, or Kaspersky, but do you use any of those -- or others -- that filter your browsing before it reaches Firefox?