In, just released, Firefox 115.0 hardware video decoding is enabled by default for Intel GPUs on Linux

While some of us had it turned on for some time, some didn't. Looks like it is fully stable now.

Yey! Test it by starting intel_gpu_top and than playing video in firefox, if row "Video" goes over 0.0% it is working!

intel-gpu-top: Intel Coffeelake (Gen9) @ /dev/dri/card0 -  921/ 925 MHz;  38% RC6
    1.78/10.41 W;      813 irqs/s

      IMC reads:     3883 MiB/s
     IMC writes:     1632 MiB/s

         ENGINES     BUSY                                         MI_SEMA MI_WAIT
       Render/3D   12.82% |████▊                                |      0%      0%
         Blitter    0.00% |                                     |      0%      0%
           Video   27.99% |██████████▍                          |      0%      0%
    VideoEnhance    0.00% |                                     |      0%      0%

   PID              NAME   Render/3D      Blitter        Video      VideoEnhance  
   578       RDD Process |            ||            ||███         ||            |
 32431       firefox-bin |▊           ||            ||            ||            |
  2301    elogind-daemon |▌           ||            ||            ||            |
  4151        pcmanfm-qt |            ||            ||            ||            |
  4153         keepassxc |            ||            ||            ||            |
  4154         pasystray |            ||            ||            ||            |
  4521             slack |            ||            ||            ||            |
  6067            chrome |            ||            ||            ||            |
 29222       jcef_helper |            ||            ||            ||            |

Of course, you need second monitor, it goes down to 0 if video is not visible, since Firefox is not rendering it.

Official release notes: https://www.mozilla.org/en-US/firefox/115.0/releasenotes/