Hello TrueBlue1981. The most recent Firefox version is 48.0.2, the minimum Firefox version for YT+ 1.4.1 and above is FF 48 due to the migration of the addon into webextension version. Please update your browser to the latest version in order to use the most recent YT+ version. You can see the required Firefox version ("Works with Firefox 48.0 and later") in the version information section at the end of the addon page, or in the version history page : https://addons.mozilla.org/firefox/addon/youtube-plus/versions/