For me chrome was always opened instead of my default firefox. I use the Appimage under Ubuntu 24 and tried:
xdg-mime default firefox.desktop x-scheme-handler/http
xdg-mime default firefox.desktop x-scheme-handler/https
but that didnt help, what did help was:
xdg-mime default firefox_firefox.desktop x-scheme-handler/http
xdg-mime default firefox_firefox.desktop x-scheme-handler/https
Hope that helps someone. No sure why the desktop files have this strange naming. Looking into ~/.config/mimeapps.list gave me the idea.