Firefox has an internal PDF viewer that is enabled by default since version 19. There have been some problems mainly with the quality of printed documents when using this internal viewer. You can save a problematic PDF by right-clicking it and selecting "Save Link As..." and then open it with Acrobat Reader. Or you can change how Firefox opens PDF files:

Firefox on Linux: Edit → Preferences → Applications → Portable Document Format (PDF) → Use Adobe Reader.

Firefox on Windows: Firefox button → Options → Options → Applications → Portable Document Format (PDF) → Use Adobe Reader.