My new Bluetooth headphones introduce a delay in the audio . . .

Contact their support. There should be no delay.

I'm not seeing this device listed in the troubleshooting information next to your question. There is a latency value present, so maybe check this in the volume controls on your Mac OS.

Media:
 currentAudioBackend: audiounit-rust
 currentMaxAudioChannels: 2
 currentPreferredSampleRate: 44100
 audioOutputDevices: [{name: Internal Speakers, groupId: builtin-internal-mic|spk, vendor: Apple Inc., type: 2, state: 2, preferred: 15, supportedFormat: 12336, defaultFormat: 4096, maxChannels: 2, defaultRate: 44100, maxRate: 96000, minRate: 44100, maxLatency: 4768, minLatency: 686}]
 audioInputDevices: [{name: Internal Microphone, groupId: builtin-internal-mic|spk, vendor: Apple Inc., type: 1, state: 2, preferred: 15, supportedFormat: 12336, defaultFormat: 4096, maxChannels: 2, defaultRate: 44100, maxRate: 96000, minRate: 44100, maxLatency: 4105, minLatency: 23}]

FedMcD and cor-el--

Thank you both very much for taking an interest in my problem.

So, to answer my original question directly, I understand you to imply that you know of no add-on or extension to Firefox that will delay the video, but not the audio, of an embedded moving picture, by hundreds of milliseconds.

Please let me know if I have misunderstood.

Also, please let me know if you should hear of such a capability.

Thanks again for taking the time to respond.

--Gil