How do you stop youtube videos from downloading automatically?

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You know when you watch a video on youtube, the video continues to download the rest of it if you pause the video. I find that this can use up quite a lot of my bandwidth. Is there any way to change youtube settings so that if you click pause, youtube will not download any more of the video? Is there maybe a program or add-on I can download to manipulate youtube settings even? I am using Firefox.