export LIBGL_DRI3_DISABLE=1

In general to say: The videos playing in the browser should be most 360p on that hardware. Everything higher would for sure not the finest enjoyment. The explanation for mpv is exactly that also: It plays not higher until you set this. But then you would have more or less the same. Full-HD (1080p) or 720p is not possible as the graphicscard is not supporting that.

When I open Iceweasel-UXP from the command line I get this error:

libGL error: Version 4 or later of flush extension not found
libGL error: failed to load driver: i915

Output of glxinfo -B:

name of display: :0
libGL error: Version 4 or later of flush extension not found
libGL error: failed to load driver: i915
display: :0  screen: 0
direct rendering: Yes
Extended renderer info (GLX_MESA_query_renderer):
    Vendor: Intel Open Source Technology Center (0x8086)
    Device: Mesa DRI Intel(R) 945GM x86/MMX/SSE2 (0x27a2)
    Version: 20.3.5
    Accelerated: yes
    Video memory: 192MB
    Unified memory: yes
    Preferred profile: compat (0x2)
    Max core profile version: 0.0
    Max compat profile version: 1.4
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 2.0
OpenGL vendor string: Intel Open Source Technology Center
OpenGL renderer string: Mesa DRI Intel(R) 945GM x86/MMX/SSE2
OpenGL version string: 1.4 Mesa 20.3.5

OpenGL ES profile version string: OpenGL ES 2.0 Mesa 20.3.5
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 1.0.16

Could someone help me with this?