I am not on a Mac and so will have a different menu system. Note that Adobe is not the only method of using PDFs on Firefox, and that you may need to set firefox to use the plugin and ensure of course that it is enabled.

• see View PDF files using Firefox’s built-in viewer

Other wise Firefox may default to downloading the PDF file instead of displaying it.

Another thought is that if you do a google search and the result is a pdf, there is often the option to view a webpage equivalent of the document. I know that is not what you are trying to do but it may sometimes be a useful tip.

The Adobe plugin to display PDF files embedded may not work properly on on Mac.

Try to disable the Adobe Reader plugin in Firefox and use the external Adobe Reader application or Preview application instead.

• Firefox > Preferences > Applications > Adobe PDF document : Use Adobe Reader

PDF files may also be found under another entry like Portable document.