Oddly, this topic seems still actual in 2018.
Some newly installed applications take over the "default" status in FF's
about:preferences instead of the per-filetype normal application set at system level (e.g. Palemoon takes over as default instead of gwenview for jpeg files, Inskape or File Roller takes over instead of Okular for pdf file.).
"Always ask" is the choice that should be used in this case and then trying to create a per-user file
~/.local/share/applications/mimeinfo.cache with the lines:
The disadvantage of the above is that it is a per-file-type setting. To have a solution for all files (as said in this answer), run:
ln -s mimeapps.list defaults.list
In case this stops working: see that the undesired application is not again selected under
about:preferences: be sure "Always ask" is selected there. Setting manually a different program (even the default for your file-type at system level, but which is not marked as "default" in the list for your file under
about:preferences) might not work.
Tested in Firefox 60.0 in Kubuntu 18.04.
Something is not right with the Firefox settings. "Always ask" and "Save file" do not make sense for files already downloaded, therefore settings under
about:preferences shouldn't affect the Downloads list. But they do. And they do in an inconsistent manner: only selecting the application marked as default is followed, switching to another application is not. To reverse that one might need to select "Always ask" and then follow the steps mentioned above.