I generally prefer to save PDF files to disk and have configured my Firefox accordingly. But there is one website (of great use for me) which uses PDF for its pages excessively. Is there a way to configure (or an extension) Firefox to save PDF from all the sites and seamlessly open PDFs from one particular site inside the tab (what is done by default to all the PDFs).
Since there is probably no extension that fits your needs exactly yet, one solution you might want to try is to just use two different profiles in Firefox. One with the PDF plugin disabled and one for the website you want to allow to use the PDF plugin.
Instructions on how to use create another profile are here: http://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles You can use multiple profiles at the same time: http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox