The plugin won't work on Firefox anymore because starting with version 52, Firefox removed support for NPAPI plugins. The developers have to redevelop the application to work with the newest standards specified by the Firefox corporation. Please keep in mind that the situation occurred for all the NPAPI plugins and there is nothing to do for the latest version. Additionally, you can remove the current version and install one that worked with the plugin and then disable the automatic update.
Here is the link with old versions of the Firefox: https://ftp.mozilla.org/pub/firefox/releases/