Often (though not always) Firefox will prompt me to "Make Firefox your default browser" when first run or when run after being closed, despite me clicking "Set as default browser". According to "KDE Settings / Default Applications", Firefox ESR is the default web browser and the default file association for HTML files. Firefox and Chromium (and I presume Konqueror) are installed. A similar laptop with similar software on it does not exhibit the problem. What is Firefox actually checking to determine whether it needs to ask "Set as default browser"?