Stop YouTube Ads with TubeStop

From lifehacker.com.

Windows/Mac/Linux (Firefox): Firefox extension TubeStop disables
autoplay on YouTube videos and blocks obtrusive overlay
advertisements. Deactivating autoplay will come in handy for anyone
who's followed a YouTube link and accidentally blared a video through
the workplace, while the incidental overlay ad "blocking" very quickly
dispenses with a new feature that a lot of avid YouTube fans aren't
happy with. TubeStop works by replacing the on-site YouTube player
with the embedded player you'd see on another site (which currently
doesn't host ads and doesn't autoplay). TubeStop is free to download,
works wherever Firefox does.