Firefox 123 has a performance bug that lowers the ambient light framerate, but it should not completely freeze the playback. (Btw, the workaround for this performance drop will come in the next version of the extension.)

Some other troubleshooting steps you could try are:
Does the video fill your screen in fullscreen mode?
Does the video or the ambient light playback freeze (or both)?
In case the video freezes, have you enabled the setting "Video > Sync video with ambient light"? If yes, try to see if disabling it helps.