Firefox 40.0.3 extremely high CPU usage on HTML 5 videos (Intel driver)

[hoplaahei at openmailbox.org]

Machine is Thinkpad x230 i5-3320M HD 4000 graphics with Intel driver

glxinfo | grep render
direct rendering: Yes
     GLX_MESA_multithread_makecurrent, GLX_MESA_query_renderer,
     GLX_MESA_multithread_makecurrent, GLX_MESA_query_renderer,
OpenGL renderer string: Mesa DRI Intel(R) Ivybridge Mobile
     GL_ARB_conditional_render_inverted, GL_ARB_conservative_depth,
     GL_NV_conditional_render, GL_NV_depth_clamp, 
GL_NV_packed_depth_stencil,
     GL_ARB_conditional_render_inverted, GL_ARB_conservative_depth,
     GL_NV_blend_square, GL_NV_conditional_render, GL_NV_depth_clamp,
     GL_OES_fbo_render_mipmap, GL_OES_get_program_binary, 
GL_OES_mapbuffer,

Firefox CPU usage is extremely high playing HTML 5 videos (150% in htop) 
until recompiling audio/alsa-plugins with:

BLKCNT_P2=on: Restrict number of fragments to ^2 aligned
BUFSZ_P2=on: Restrict buffer size to ^2 aligned

This reduces CPU usage to around 25%, which is tolerable.