http://www.ghacks.net/2013/08/07/how-to-get-click-to-play-working-in-firefox-23-or-newer/
Previously, Firefox can be configured to run plugins on demand by changing a setting in about:config (plugins.click_to_play). In Firefox 23, plugins appear to run by default even with this setting. This article teaches how to get the run-on-demand behaviour back.
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