I also had this happen trying to open a PDF in Firefox recently.

brudgers · on Sept 22, 2017

It will happen if:

  about:preferences#applications ->
    Portable Document Format (PDF) ->
      Use firefox

is set. A fix is to set

  about:preferences#applications ->
    Portable Document Format (PDF) ->
      Preview in firefox

The problem can be caused (in Ubuntu 16.04) by selecting Firefox as the system default for PDF viewing.