Is this a link in an email message in Outlook? This problem comes up from time to time.

While Firefox usually can set itself as the appropriate application to open URLs you click in other programs, the settings sometimes become scrambled or do not take. This article has tips for fixing that problem: Setting Firefox as the default browser does not work - What to do. Does anything there help?

If the problem persists, is there anything special about the way you start up Firefox? For example, do you have it launch in a security "sandbox" or virtual machine? Do you have any command-line options set to allow multiple simultaneous Firefox sessions, or to call up the Profile Manager? If none of that sounds familiar, the answer probably is no.

Does that pop-up have a check-box where you select Firefox to store/save this choice (i.e. make Firefox the default)?

Solved...thank you!