This is a browser security block, not a plugin error. Firefox ESR versions after v60 will display this even if v27 is installed. Use Firefox 52.9.0 ESR or add the website to an exception list via about:config → plugins.http_https_forced_allowlist .
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Flash Player 27 is outdated and contains known security holes that hackers can exploit to hijack your system or steal sensitive data. This is a browser security block, not a plugin error