I have developed a Flutter website and use Firebase for my storage and hosting. Now I want to use two domains "www.mywebsite.de" and "www.mywebsite.fr". Can I do this with Firebase?
Yes, you can configure multiple custom domains for a single Firebase Hosting project. In fact this is quite common.
Just keep in mind that all domains will be serving the exact same content, unless you also set up multiple sites on the Firebase project.
So given your use-case, you may either serve the same resources to French and German users, or you can configure a separate site for each in the same project, and then serve different resources to French and German users.