How to modify firefox image viewer

[joe at actionline.com]

.
When I use Firefox to view a web mail service 
and I write a new  email message onto which I 
add an attachment (jpg image), when I click on 
that image to make sure that the image displays 
correctly, a dialog box pops up that says: 
"What should Firefox do with  this file?"

The options are to Open with [__The GIMP ___]v
or Save to Disk.

When I click on the down arrow "v" to choose a 
different option than GIMP (the default option), 
and then click on "Other", Firefox opens yet another 
dialog box in which I can locate a different viewer 
(i.e. 'kview') to add as an option. That all works 
fine, except ...

I cannot find where among Firefox's various menues 
and options to change the (default) from GIMP to kview.

How (where) can this default option be changed?