I wanted to be able to install a single RPM package again, so today I right-clicked on a RPM package, and chose 'Open With' > 'Other...', then entered
Code:
xdg-su -c "/sbin/yast2 sw_single %F"
This created 'xdg-su.desktop in ~/.local/share/applications. I renamed this file to software-installer.desktop and edited it so that the name made more sense to me
Code:
Name=Software Installer
There should be corresponding update in ~/.local/share/applications/mimeapps.list as well.
Now when I want to download a single RPM package, FF gives me the option to install the file using 'Software Installer' as desired. :)