Adobe Acrobat NPAPI Plug-in has been blocked for your protection.

Why was it blocked?
The Adobe Acrobat plugin doesn't work on Mac OS X in default (64-bit) mode, showing a blank page when users click on links to PDF files in Firefox. It also causes a significant number of crashes in 32-bit mode.

There's more information on this blog post.
Who is affected?
Firefox users on Mac OS X who have installed the Adobe Acrobat plugin.
What does this mean?

Users are strongly encouraged to disable the problematic add-on or plugin, but may choose to continue using it if they accept the risks described.

When Mozilla becomes aware of add-ons, plugins, or other third-party software that seriously compromises Firefox security, stability, or performance and meets certain criteria, the software may be blocked from general use. For more information, please read this support article.