Hi, that page has a tag listing a URL for composing new messages, and since Firefox has detected that Outlook isn't one of your options for handling "mailto" links (like [mailto:bob@example.com mailto:bob@example.com]), or that a previously saved compose URL does not match, it is offering to add it. For more information about where this URL goes, see: [[Change the program used to open email links]]. The easiest way to stop getting the message is to add the URL to your mailto options. A different approach was suggested in another thread, but I haven't tested it myself: https://support.mozilla.org/questions/1445518