Try setting the executable in System Settings->Default Applications->Web Browser.
Make sure “in the following browser:” is checked and enter firefox in the input field.
System Settings is a different application, by default it’s in your favourites in the application launcher (left-bottom corner).
If it’s not you can find it using the search bar at the top of the launcher.
In my Desktop folder ("$HOME/Desktop") there is a file with name “MozillaFirefox.desktop”. It is the only thing I am aware of that uses the name “MozillaFirefox”. Perhaps Kontact uses that to launch a browser. You might check whether you have that file, and if you have it, whether clicking on it launches firefox.