Reported here: https://github.com/mozilla-mobile/fenix/issues/13153#issuecomment-775987632

Relevant part of logcat:

02-09 17:58:06.465  8343  8492 I Gecko   : [GFX1-]: Failed to compile shader: composite
02-09 17:58:06.465  8343  8492 I Gecko   : 0:5: P0003: Warning: Extension 'GL_OES_EGL_image_external_essl3' not supported
02-09 17:58:06.465  8343  8492 I Gecko   : 0:9: L0001: Typename expected, found 'samplerExternalOES'
02-09 17:58:06.465  8343  8492 I Gecko   : 
02-09 17:58:06.465  8343  8492 E webrender::device::gl: Failed to compile shader: composite
02-09 17:58:06.465  8343  8492 E webrender::device::gl: 0:5: P0003: Warning: Extension 'GL_OES_EGL_image_external_essl3' not supported
02-09 17:58:06.465  8343  8492 E webrender::device::gl: 0:9: L0001: Typename expected, found 'samplerExternalOES'
02-09 17:58:06.467  8343  8492 I Gecko   : [GFX1-]: wr_renderer_render: Shader(Compilation("composite", "0:5: P0003: Warning: Extension \'GL_OES_EGL_image_external_essl3\' not supported\n0:9: L0001: Typename expected, found \'samplerExternalOES\'\n"))
02-09 17:58:06.468  8343  8372 I Gecko   : [GFX1-]: Fallback (SW-)WR to Basic

This device supports GL_OES_EGL_image_external, but not GL_OES_EGL_image_external_essl3. (about:support provided here.) When we composite the video we attempt to compile the composite_TEXTURE_EXTERNAL shader, which fails because of the lack of extension support. Presumably if we did not promote the video to a compositor surface, we would fail in the brush_image_TEXTURE_EXTERNAL shader instead.

Ah, this is a duplicate of bug 1507074