Problems with Mate

[Bill]

Hi,

I don't know how it happened but whenever I mount a partition, Firefox 
is opened and the contents of the partition is displayed in Firefox.  I 
have to close Firefox and open (display) the partition with Caja which 
is the proper application to brows a partition.  Recently, the same 
problem exists in Trash but there is no way to display the contents of 
trash with Caja (which is the proper program to open trash).

I see no way to change this in System/ Preferences/"Preferred 
applications".  Where can I reset the preferred application from Firefox 
back to Caja, which is the default?

Bill Stanley