Firefox : Failed to initialize WebGL

[JB Data31]

Hi,

My env is an ubuntu VM guested in VirtualBox.

Using OpenGL from command line works fine (see rotating cube screenshot).
$ glxinfo | grep "OpenGL version"
OpenGL version string: 3.1 Mesa 20.2.6

But using OpenGL in FireFox (https://get.webgl.org/) fails with log:

Failed to create WebGL context: WebGL creation failed:

  * tryNativeGL

  * Exhausted GL driver options.

I have set webgl.force-enabled to true in FF params.

I have installed libdrm-dev with apt-get.

Issue remains...

Help appriciate.

JB.