Where: Firefox in Linux Mint Mate
Problem: Nautilus is invloked instead of Caja
How: Right clicking a downloaded file from the downloads dialog and selecting 'Open Containing Folder' Triggers it.
Solution: in mate menu look for preferred application under preferences menu
If the drop down box is blank, reselect mate file manager.
These solutions below did not work or might of helped? Source
create two bolean strings and set both to TRUE
Create a string and add 'caja' to this