You should update your Firefox 15.0.1 to the latest Firefox 16.0.1 so that security issues known in prior versions may be avoided
PDF Problem -
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What did not work?
If item 2 above did not work:
Some additional information here - https://support.mozilla.org/en-US/kb/view-pdf-file
I'm having a similar issue - pdfs won't load. I'm running firefox 16.0.2 on Mac OS 10.6.8.
BTW, they load on Safari, but they won't save or download.
Disable the Adobe Reader plugin in Firefox (Tools > Add-ons > Plugins) and use the external Adobe Reader application or Preview application instead.
PDF files may also be found under another entry like Portable document.
Check this article out, Firefox blocked the Adobe NPAPI plugin from working because it is not supported in the new version of OSX, and causes crashes in 32 and 64-bit...
The work around is to go to your preferences and change what Firefox does when you click on that file type from using the NPAPI plug-in to instead using the default Adobe Acrobat Plugin -- it actually still opens the PDF in your browser without opening a separate Adobe window -- looks great.
(scroll down to step 4)