Make sure that plugin is enabled.

• type about:addons in URL and extension > choose the plugin and make sure that plugin is enabled and updated.

https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

The plug-in was in fact still enabled and was never disabled.

1. Quit Firefox
2. Remove the QuickTime Plugin from /Library/Internet Plug-ins (move it -- don't delete it)
3. Start Firefox and load any PDF. They should now be handled by the Schubert PDF plug-in)
4. Quit Firefox (not sure if this is necessary)
5. Move the QuickTime Plugin back to the Internet Plug-ins folder.

http://hints.macworld.com/article.php?story=20050729102136356

We have received numerous reports from our customers using Firefox 18 for Mac that the Schubert PDF Browser Plugin does not work.

Is there a different PDF Browser Plugin that will wok with it and work well with all websites?

Firefox has PDF viewer inbuilt

If Firefox has a PDF viewer built in then it doesn't appear to be working in Firefox 18 for Mac. Our users are complaining that they are simply being shown the file path of our DLL and not the PDF that streams as an application/PDF from that DLL.

They may be using some content or ad blocking extensions if they do not get the file.
I assume that the DLL runs server side and is used to retrieve a specific document.
Checking the server logs should confirm that you get a request to retrieve a non existing file.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes