Firefox wont open/display a pdf file. It may prompt to download the file.
This may happen if no PDF viewer is installed, or if firefox is wrongly configured.


Install pdf viewer

Install a pdf viewer like Adobe reader or Foxit reader.

Update pdf viewer

If you have updated Firefox, your PDF viewer maybe needs to be updated too.
Go to Tools->add ons and check for updates.

Associate pdf type with viewer

Go to Tools->Settings->Applications. Change pdf file to be openend with a pdf reader you installed.