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/