In 1330729 Caitlin's PDF viewer setting got somehow set to Adobe. With NPAPI plugins disabled, in-browser PDF preview was just showing the disabled Adobe plugin. It is unknown right now how many users are affected by this, but since this probably was triggered by installing Adobe there is a risk that many machines are corrupted out there. Possible next steps: - Run an analysis on how many users are affected - Fix the preference for users with "broken" setting - Disallow incompatible plugins to be set as viewers for any content type

Benjamin, should we reset all content type settings in about:preferences#applications that point to NPAPI plugin handlers?

Having read through bug 1330729, I don't think that there is an issue here, but I'm going to ask our plugin QA lead to take a look. In particular, the problem Caitlin had was that we still recognized the plugin, but it wasn't working properly with click-to-play (a known issue with full-page plugins and things loaded from file: URIs). With the removal of NPAPI, the browser won't even recognize that there is a plugin, so the former setting won't even be a thing. Stefan, here's the STR to check. If you could report back what the actual experience is and whether you see any obvious bugs: A. Install Firefox 51 and Adobe Acrobat Reader with the NPAPI plugin. B. In Options / Applications / Portable Document Format (PDF) Choose to view PDFs in-browser C. Visit a PDF such as https://www.irs.gov/pub/irs-pdf/f1040.pdf . Firefox should prompt you to activate Acrobat, and after that it should hopefully work. D. Upgrade to Firefox 52 (aurora or beta) (with the load_flash_only pref set to true) E. Reload that same PDF file from step C. I believe it should either automatically revert back to using the builtin PDF player, or it should prompt you to download the file. F. Go into Options / Applications again and see what option is selected for PDF files.

Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:52.0) Gecko/20100101 Firefox/52.0 In version 51 I`m prompted to "Activate Adobe Acrobat" and the PDF is open inside the browser. I`m able to navigate within the document and fill out all the forms in it. According to me in 51 is working perfect, my experience is excellent. In version 52 the PDF setting is reverted to "Preview in Firefox". I'm able to navigate through the document, but unable to fill out any field. In the same time I`m prompted to download the PDF file locally or open it with "TWINUI" (by default) which opens Microsoft Edge where I`m still unable to make any changes to the file. If I choose "Other..." and use the "Adobe Acrobat Reader DC" instead the file is editable and opened inside the Adobe application. I definitely prefer the behavior in version 51. No obvious bugs found during testing.

Too late for firefox 52, mass-wontfix.