Local flashes not playing offline in Firefox? Here's how to fix it: 1) Type "about:config" into your address bar and search for "plug*ht". 2) Double-click "plugins.http_https_only" to set its value to false.
Flash NPAPI plugin not supported in Firefox? Since Oct2017 you may need to run an Extended Support Release, even though they said it wouldn't happen until 2020. You should be able to install into your current Firefox folder to upgrade but existing plugins from before upgrading might screw things up in the ESR version. Uninstall, clean and reinstall the ESR version if the browser starts acting weird. Clean means manually deleting folders Firefox leaves behind. Navigate to %appdata%. Paths: AppData/Roaming/Mozilla, AppData/Local/Mozilla and AppData/LocalLow/Mozilla. Update Nov2017: Flash now seems to work in Firefox 57. I'm not sure what's going on.