In Ubuntu, the shipped Firefox 15 comes with the new embedded HTML5 PDF reader disabled.
It is easy to enable though.
Go to “about:config”, search on “pdf” and switch the “pdfjs.disabled” entry to “false”.
Restart Firefox and test it pointing to some PDF URL.
In case it does not work, have a look at the “Portable Document Format (PDF)” type to action association in “Edit->Preferences->Applications” and set it to the internal Firefox preview.