Solved - didn't see "Personal Use only Box" below "send for License" button.

Spoke too soon - Every time I close and re-open Firefox, the Acrobat NPAPI Plugin, Version 11.0 has to be disabled. I don't want to delete it because it may be useful for other browsers. What can I do?

I had Acrobat Pro 9 and went to the check add ons, which told me that I should update now. I did and Reader was installed. I can't exactly remember what happened after that, only that I wound up installing Acrobat XI and, even after trying to configure Firefox and Acrobat, some other application was displaying PDFs in the browser so that I always got an error message and an option to use a different application (like Acrobat). But when I chose that, instead, it would download the file, not open it in the browser.

I checked the Applications option to ensure that Acrobat was supposed to open in Firefox. Check. I checked to see if Acrobat was enabled to open PDFs in the browser in the Add-On screen. Check. I disabled the PDF Viewer (pdf js). While that solved the annoying problem of seeing the uglified PDF file, it would only download the file. [See https://support.mozilla.org/en-US/kb/view-pdf-files-firefox-without-downloading-them for some of what I did. I was too chicken to have Firefox reinitialize all my plug-ins.]

Oh, and I did disable DJVu Plugin Viewer as well.

Bottom line: none of the fixes worked to make PDFs open in Acrobat XI in Firefox. It works fine (albeit slowly) in IE 9.