How to have Firefox save txt files automatically instead of opening in browser

How to have Firefox save txt files automatically instead of opening in
browser

When I click on a txt file I would like the browser to save it to the
default download folder instead of opening it in the browser directly.
I have checked Options --> Applications to determine how the browser
handles text/plain content types and beside "text document" it says
"Save".
In the mimetypes.xml file under the current firefox profile, this entry
refers to text/plain documents, so it should save the files automatically.
However it is still being opened directly in the browser. Is there a
different setting that I need to change? I know with PDF files, firefox 18
or something came with a special pdfjs extension that overrides my own
settings and had to be disabled in the configuration, but not sure if
there is something sneaky like this for plaintxt files.
I tested it on links to txt files from http://www.textfiles.com/history/
I am using the latest version of Firefox.