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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.