virtualbox.org

My both guest and host are ubuntu, but only on the guest, Firefox cannot play twitter videos. error:
We cannot play the video in this browser. Please try a different web browser.

Guests tested: ubuntu 18, ubuntu 16

Most probably your browser expects some video acceleration that's not readily available, either because you didn't install the GAs, or you haven't enabled 3D acceleration, or the level of 2D/3D acceleration provided by VirtualBox is not adequate enough.