In some non-YouTube webpages there are YouTube embedded videos. Usually pressing a "play" button within them will make them play. Associated with each of those videos there are several thumbnails, one of which is the "default".
I am looking for a Firefox extension (though I suspect it doesn't exist) that, when Firefox loads the page, makes it so that instead of getting a webpage with an embedded YouTube video, I get instead the same webpage with the default thumbnail for that video --- as an image, not as an embedded video --- in the same place (and taking up the same space) as the embedded video used to have.
In other words, is there a Firefox add-on that replaces an embedded YouTube video with its default thumbnail?