I'm using Windows 10 + "Dolby Access" (Atmos) app with only "volume leveler" turned on (Custom 1 on left side of Settings). Then turn on Atmos headphone virtualization for your current audio output (right click the speaker in system tray, then switch from "off" to "dolby atmos for headphones".
Using a free latency estimator (web page on github) I recorded a good amount of additional latency (can't remember if it went from 130 ms to 230 ms or 330 ms, but it was a good amount longer). When I play MKV files locally in MPC-HC the latency is queried and used to make the video timing match the audio. This isn't happening on a number of big websites (IMDB, YouTube, etc). I've reported it to them, but wondered if Firefox was to blame.
Before I wrote a bug for Mozilla, I thought I'd mention it here and see what someone else thinks. I assume each website is using their own video playback software and DRM. But if they reuse broken code through Firefox there isn't anything they should do to fix the issue.