The codec of the video may be limited to that of the device? In order to further investigate:

1. What is the device type?
2. What is an example of a url where this is happening?

Thank you, this will help an investigation.

puppyd0g1 said

Firefox doesn't play any videos on theverge.com and instead displays "this video is not encoded for your device" message. Please look into this.

I am using nexus 9 lollipop 5.0.2 and here is a link to a video I can't play http://www.theverge.com/2015/5/7/8561465/best-ipad-stylus-pen-sensu-adonit-wacom-review

Hi puppyd0g1

Yup it's a problem in Firefox for Android 37 and 38 beta. It seems to be fixed in FF40 nightly. In other words if you go to nightly.mozilla.org and download Firefox for Android nightly version 40 and the Flash Player addon and request the desktop site (Firefox for Android Menu | Request Desktop Site) it works fine on the Nexus 9.

Curiously I don't seem to have this problem on Firefox for Android 37 on a Samsung S6 Edge. I will investigate further and file a bug (even if it turns out to be a Verge issue and not a Firefox issue)

In the meantime, as a workaround, you can go to the Verge YouTube channel and play the videos from the YouTube app.

Cheers!

...Roland