WPFV can't be selected using the browse dialog. This is because the WPFV itself is a DLL file rather than a .EXE.
FYI, here is the command-line for WPFV
rundll32.exe C:\WINDOWS\system32\shimgvw.dll,ImageView_Fullscre en <filename>
Only option is to set WPFV as the default viewer in Windows, so that it's listed automatically in Firefox' Browse dialog box.
Try fixing the JPG association using one of the fixes:
Repair image file type associations in Windows XP:
Repair image file type associations in Windows XP
File association fixes:
Windows XP File Assocation Fixes