Pause autoplaying HTML5 videos on load. In Firefox just go into about:config and look for media.autoplay.enabled and set to false.
I'm using this 'clickhappier-archive' account to post archival copies of old MTurk-related userscripts from userscripts-mirror.org (which is itself an archive of the defunct site userscripts.org) and forum posts, which their authors or others haven't re-posted on Greasyfork yet. Some of these may no longer be all that interesting as more recent scripts may do what they do better, but some remain very useful.
This script was archived from http://userscripts-mirror.org/scripts/show/119838
Author: Scuzzball
Script Name: Prevent autoplay from HTML5 Videos / Pause all HTML5 videos on load
Last update Apr 30, 2012 — Installed 4,057 times.
Script Summary: Pause all youtube [and other] HTML5 videos on load.
(And as described by DCI in this forum post): [Certain HITs with embedded audio/video] always load slow. I think it's best to do them in two tabs, but in order to do that you need to disable autoplay. In ff [Firefox] you can do that by going into about:config and setting media.autoplay.enabled to false. Chrome doesn't have that option so you have to use a userscript. This will disable autoplay in Chrome (or FF, but not perfect or necessary).
Version: 1
Note from Clickhappier: I successfully use this script in Chrome (non-HTML5 videos can't be automatically stopped like this, alas). Between NoScript, FlashBlock, and the about:config setting, I don't need it in Firefox, but it causes no issues there either.
For many more MTurk-related userscripts, see the Turking Scripts set.
