Sounds like an issue with the "Free YouTube to MP3 Converter" software you installed on your computer. It might help if you post a link to the website where you downloaded this software so that someone here can take a look.

P.S. You said, ''Free YouTube to MP3 Converter starts Microsoft Edge although Firefox is defined as Browser.'' I took that to mean that the "Free YouTube to MP3 Converter" program has a setting to choose the Web browser that opens. If you meant that Firefox is set as the default browser in your Windows program settings, it's possible that a Microsoft Edge or Windows update reset your default browser and you have to set it back to Firefox. See [[How to change your default browser in Windows 10]]. You also said, '' Only an Uninstall of Microsoft Edge ends that behaviour. But after next update Edge is again installed.'' I don't know if you can actually uninstall Microsoft Edge from your computer since it comes with Windows 10. See https://support.microsoft.com/en-us/help/4576988/why-can-t-i-uninstall-microsoft-edge and https://answers.microsoft.com/en-us/microsoftedge/forum/all/how-do-i-remove-microsoft-edge-from-windows-10/860cd7b9-f903-4386-a71a-84f94a110c46 You can get more help with that on a Windows forum such as https://www.tenforums.com/ or https://answers.microsoft.com/en-us/windows/forum/windows_10 ... or the Microsoft Edge forum at https://answers.microsoft.com/en-us/microsoftedge/forum