Is it FireFox version 43?
If yes, then let me inform you that this is a known issue and we are working on it.
We plan to provide support in future updates.
Meanwhile, as a workaround, you can use Adobe Create PDF extension for Chome and Internet Explorer, or use Acrobat for converting web pages to PDFs.
I'm having the same problem. FireFox blocks the plug-in and the alternative is to use another app that sends the page to their site for conversion. It adds an overlay with the company's name and if you don't like that, you can pay for the service at pdfcrowd.com.
So, what's up, Adobe? You used to be the toast of the internet with Acrobat and Flash and now everybody blocks you. It really diminishes the value of an Adobe product. Seriously.