Firefox 19 comes with a new PDF viewer add-on called PDF.js; it is absolutely terrible! The first PDF I tried to display (http://helpx.adobe.c...r_reference.pdf) looks like this
To fix this and use your standard PDF viewer plugin: Tools | Options | tab Applications
change (Content type: Portable Document Format)
to
Now your PDF document should display correctly in your PDF viewer plugin
In case you do not see 'Portable Document Format' under the Applications tab, you may need to reset your Firefox profile (Help | Troubleshooting | Reset Firefox).