Menage plugins permissions: block or allow.
Note: plugins.click_to_play in about:config (“Block plugins” checkbox in button's context menu) should be enabled.
Unfortunately since Firefox 20 (Gecko 20) global exclusions doesn't work, only on per-plugin basis.
So you should change "Shockwave Flash" and "plugin:flash" in the source (and create copy of this button) to menage other plugins.
