I'd start researching here: [Link: wiki.mozilla.org].

By the way, Firefox prompts the same way. In the options box, I select open with and in the drop down box it defaults to "Harmony.hol" or "Harmony.exe". Sorry, but I can't remember which extension. I do recall that once I installed firefox it originally thought the appropriate open with file was outlook.exe... I uninstalled and reinstalled Harmony software and issue was fixed.