Thanks.

I use BROWSER environment variable for that.

It seems to even override XDG settings.

    % xdg-settings set default-web-browser firefox.sh.desktop
    xdg-settings: $BROWSER is set and can't be changed with xdg-settings
    
    % echo ${BROWSER}
    firefox

Hope that helps.

EDIT: You also got me nice idea - to add http(s):// and ftp:// support for mine see.sh opener :)