Hi Marguerite, sorry about that. The usual reason for that message is that installer/updater didn't replace all of the program files so there is a mix of different version files. You can try downloading and re-running the installer and hopefully that will not happen again. However, if a file has been locked by another process in your computer, you might need to do a regular shutdown and restart of Windows before it takes. You can download a full installer from: https://www.mozilla.org/en-US/firefox/all/#product-desktop-release