Is this what you're seeing? see screenshot I always see it. You just click the x on the far right or just ignore it and it will go away. It's an option in your settings. It's asking do you want to set Hotmail as your "mailto" option. Here's more info on the option Mailto below. It's a link to google search results. https://www.google.com/search?client=firefox-b-1-d&q=mailto

The *outlook.live.com* site is not unknown to you though, you can safely add it as a mailto handler and it will stop asking. If you don't ever want to add a new mailto handler, you can change network.protocol-handler.external.mailto to false in about:config.

Thank you both for responding. Much appreciated.

Mark it as resolved if you have no further questions and have a nice day.