How to change Firefox file opening defaults?

[Josef Lowder]

.
I think I asked this once before, but I have not yet found an answer or
solution  to my question. 

How can one change the defaults (and add new options that will stay there and
not disappear at the next startup) in the Firefox file opening dialog?  An
example is shown at this link: http://www.firefox.com/temp/open.pdf.jpg 

The default was 'xpdf' but then after I installed Acrobat Reader, that became
the default and 'xpdf' disappeared from the list.  I added 'xpdf' back it is
no longer the default.  How can I make xpdf the default again? 

Also, when I asked this question previously, it was pertaining to image files
rather than to pdf files, and in that case, the default was Gimp.  With the
"other" option, I was able to add 'kview' but I was not able to make it the
default.  And, sometimes, 'kview' has just disappeared and is no longer even
an option, so I have to add it again and again. 

I am using Firefox 1.0.4 (because I'm apprehensive about switching to a newer
version of Firefox (another story).  In my version of Firefox, "Preferences"
does not allow (in any way I can find) for these changes to be made.  Surely
there are configuration files in which one should be able to make the necessary 
changes, however.