Hello there,

Lets try to fix this. Follow the following steps:

1.  Click the menu button in New Fx Menu and choose Options

2.  Select the Applications panel.

3.  The Applications panel will display. Select the type of file for which you 
      want to change the default action. (in your case DJVU-file)
 
4.  The Action column will give you a drop-down menu, with options on 
      action to take, whenever you click that type of file. 

https://support.cdn.mozilla.net/media/uploads/images/2014-08-09-07-28-26-cb20cb.png

       Always ask: will prompt you to select what action you want Firefox   
        to take when you click on that type of file. This can be useful if Firefox 
        is automatically saving a file type or is always opening it with a certain  
        program and you want to be asked what to do.

        Save File: will always save the file to your computer using the 
        Downloads window, whenever you click that type of file.
        Open the file with an application or plugin of your choosing. 

5.  Click OK to close the Options window after making changes. 

You can also refer to this article https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file for more info.

Hope it helps.

Thank you for your reply!

Well, this is not precisely what I wanted to do: my option in the Application settings is set to "Always ask", and this is what I would like Firefox to do. However, after the file is saved, I expect that it is opened with my system's default application for DJVU files, but Firefox just keeps opening it with the text editor (gedit) if I click on the file in the download drop-down menu in the toolbar.

Meanwhile, I have figured out how to fix it, however I find that this should work from the beginning without having to fix it.

The problem was that in the application settings there was no default application set for that file type, although my system specifies one (see first picture).

After I have selected "User other..." and set it to my preferred programme for DJVU files (evince), the menu has changed (see second picture). Now, even if I leave the option "Always ask" set, the correct application is used for opening that file type.

There are several questions which I am wondering about: 1) Why is this not set to system's default application if the user has not manually specified anything else? I think using the text editor instead is not the best option. 2) Why is the behavior in the downloads window (="Library") different from the drop-down menu? Even with the default programme not set in the applications preferences, clicking on the file in the downloads windows opens system's default programme.

I am not sure if this is the desired behavior, for me it seems slightly inconsistent.