I am using dolphin as my file manager and when I download something via firefox, I right click the download dialog and choose "open folder", I always get nautilus.
Why doesn't firefox open the folder with dolphin?
Note: I tried adding the tag "dolphin" but because of my low rep I can't create new tags 🙁
To get Dolphin in the file-picker dialog, install the package "kmozillahelper" from the standard repositories.
Set the filepicker-variable in in Firefox by visiting the URL about:config.
More information here