pages keep saying I need a plugin but "search for plugin" button CONSISTENTLY yields "none found" -- do I need one or not?

I have a nearly-new Mac 10.7.5. Downloaded Firefox just fine. But many (not all) pages give me a popup saying that additional plugins are needed to display all the content. When I click the "Install missing plugins" button, I get a popup saying, "No suitable plugins were found." Does this mean the popup saying I am in need of a plugin is false, or what?

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