Since this topic was closed, without answering my question:
I will do it via this way. By default Firefox opens the program Catfish within Manjaro XFCE, when the download is finished and you want to open the destination. Same is applicable for other applications like Transmission.
Nevertheless, the original topic point out the solution to change the MIME default of
inode/directory to be used with the file browser (=Thunar), which is fine using XFCE!
So why is Manjaro by default configured this way? Still even in Manjaro v18.
I really do not see any benefit of configuring MIME inode/directory to open with Catfish?
(Yes, I do know what Catfish is, it's a graphical search app for your file system).
Maybe it's a wise idea to change the default MIME inode/directory of Manjaro, atleast regarding XFCE to use Thunar?
Melroy van den Berg