Did you check the download actions ?

• Settings -> General -> Applications
• https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file

I tried, but somehow I can't choose "open in Firefox" option for mp4 files. Even I choose Firefox from the 'select other apps' menu, it's not applied.

When I set the option to "ask every time", I can choose Firefox in the pop-up window after clicking mp4 links. But in private mode, it's opened in non-private window...

Thank you for your assistance.

Maybe look at an extension:

• https://addons.mozilla.org/firefox/addon/content-type-fixer/
• https://addons.mozilla.org/firefox/addon/open-in-browser/

Are you able to play this video directly in the browser?

https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4

Both add-ons didn't work. And I couldn't open the Mozilla's example video directly in the browser either.

I tried on another Firefox in different Laptop, and mp4 links could open in Firefox. It seems opening in Firefox function in my Firefox is broken...

And I found windows on the PC is broken, too. (I can't Windows Update) So I'll re-install OS, and Firefox too.

Thanks anyway.