When the video is playing, does Firefox show a speaker icon up on the right side of the tab? That is the audio playing indicator and mute button. If that doesn't show, for some reason, the audio is not getting from the page to Firefox. Assuming you have the audio playing indicator and it's not slashed (muted), check the Windows 10 Volume Mixer. To open that, right-click the little speaker icon in the Windows Task Bar's notification area and click Open Volume mixer. Each app sending audio to Windows should have its own slider. Does Firefox have a slider? Note that in some cases, they may be a horizontal scroll bar to see all the app sliders. If Firefox has a slider, is it showing movement to indicate the current volume? If there is movement, but it's grayed, check the mute button at the bottom of the slider. Assuming all looks good in the Volume Mixer, the other issue is that Firefox sometimes directs output to a previously connected device that has been removed. I don't recall the fix for that. (Some of the above overlaps this article: [[What to do if Firefox won't play any sounds]])