Could you check something: in the tab where you want to play the video, first open Firefox's Network Monitor, one of the Developer tools, in the lower part of the tab. You can do that using either:

• menu > Developer > Network Monitor
• Ctrl+Shift+k then click "Network" along the top edge of the web console

Now try loading the video in the upper part of the tab. Cancel the download.

The Network Monitor should show the request. If you click it, usually Firefox will show "Headers" and among these should be the Content-type. Does it show video/mp4 or some other content type? If it's not video/mp4, Firefox might not think it can play it.

The headers might also include Content-disposition. If that is there, does it say inline or attachment? (Attachment forces a download, inline should play in the tab.)

Thanks jscher2000. I checked your suggestion. The Content-type shows 'application/octet-stream'. I should add that I'm using a private platform by Amazon to upload the videos.

I have solved half the problem by changing the value of media.play-stand-alone to true. However, I'm uploading two kinds of mp4 videos. Upon checking both on the freeware software 'MediaInfo', I can say that the ones identified as M4V (Codec ID) are now playing in FF, while the ones identified as isom still try to download.

Maybe is a matter of keep changing values in the About:config tab, but I don't know what else to change yet.

CarlosMdq said

The Content-type shows 'application/octet-stream'.

Can you modify the server settings to send video/mp4 ?