That is either a browser thing or an Appley thing. For me (PC) in Chrome it always opens a save as window. In FF it opens a window with options to either open with a selected software or save as. It also has a tick box to “Always take this action in future”. None of this can be influenced by kiwitrees.
Strange. FF opens a window with the CSV, no option to save as a file. FF Prefs cannot be modified as to what action to take for a file type. Forget the reason they opted for that.