This concerns 13.1 LXDE. I've installed Firefox and uninstalled Midori because it didn't work
correctly with some websites. There's a script that looks for
Midori and gives me errors because it's not found.
Where do you get that message? There is a BROWSER environment variable that points to midori. That's probably it, it is set in /etc/profile.d/lxde.sh. You can remove the file completely if you don't have midori.