You made no mention of this : https://support.mozilla.org/en-US/kb/applications-panel-set-how-firefox-handles-files Please check the settings for PDF and Click it to change it.

Please let us know if this solved your issue or if need further assistance.

Hi bmakool, it's true that Firefox no longer runs PDF viewing plugins (since Firefox 52). If a site cannot let you see the PDF without a plugin -- just using the built-in viewer -- then you may not be able to view that site's PDFs in a tab. But I wouldn't expect to see that message on a lot of sites. Is it very widespread in your experience?

Pkshadow, I'm editing this reply after my problem was solved because I realized that you, too, DID direct me to the solution, but I didn't understand at the time which option to pick under Applications for Portable Document Format. jscher2000 clarified for me that I needed to pick the Preview in Firefox option. Thank you again!

Thank you, Pkshadow. I had to use the link for the newer version of Firefox (57), and I did select the Adobe Acrobat DC (default) option, but it didn't fix my viewing problem.

Thank you, also, jscher2000. I have only been dealing with one site so far. It's a Thinkific online course site that I use to create courses, and I have always been able to open and view PDF files on it via Firefox until yesterday. Firefox did updates on my laptop yesterday or the day before, and I'm wondering if that could be part of the problem.

Does either of you know if there's a simple way to return to or reset the Firefox built-in (default) viewer? Perhaps that's what I just tried with Pkshadow's suggestion above? Obviously, I'm not Tech savvy!

So do you have something like adobe reader or such installed on it?

bmakool said

I did select the Adobe Acrobat DC (default) option, but it didn't fix my viewing problem.

That means you want any PDF download to open outside Firefox in Acrobat.

To try to open a PDF in a tab, select Preview in Firefox.

I have Adobe Acrobat DC installed on my computer, and the PDF files were opening just fine on the Thinkific site (via Firefox) for the past two years until yesterday. The only thing that I changed a few days ago were my security settings on Firefox to try to prevent sites from tracking me all over the Internet. I undid those security changes today, but it didn't fix the PDF viewing problem. I'm wondering if Firefox updates could possibly have messed something up, but if so, I'm sure there would be other users having the same problem.

Thank you, jscher2000! I will try that when I get home from running errands and let you know if it fixed the problem.

Tools / Options / Files and Applications / Applications / Search for: pdf

Portable Document Format (PDF) / Action / . . . Change it from "Preview in Firefox" to . . . "Save File" ? No, then it just downloads the file . . . Use Other / Select Foxit Phantom - then it auto-opens the PDF - this saves a step

You could have it open in a tab in FF for preview, but it's said that this could "damage" the PDF

Thank you for researching this issue, sludge7051-x. I just returned to my computer and, after trying the suggestion from jscher2000 above (to select "Preview in Firefox" in the Applications section), I am very happy to report that the PROBLEM IS NOW FIXED, and I can open and view PDF files again in the Thinkific course site.

I greatly appreciate the time that you all spent to help me with this issue. THANK YOU!!

sludge7051-x said

You could have it open in a tab in FF for preview, but it's said that this could "damage" the PDF

Firefox renders the PDF as a canvas with an invisible text layer. So it doesn't have perfect fidelity to the original; sometimes it's much worse than perfect. But if you use the Download button on the black toolbar above the PDF to open or save the PDF, you should get the original file.

Somewhere I read that you shouldn't open PDFs in a browser, because that can corrupt the PDF (no idea how) . . . so, I have always just downloaded them, and then opened them in Foxit Phantom. I was using Foxit Reader Free before that . . . are you saying that this is no longer an issue, at least with FF?

sludge7051-x said

Somewhere I read that you shouldn't open PDFs in a browser, because that can corrupt the PDF (no idea how) . . .

I don't see how they could be corrupted with the built-in viewer. Just don't use Save Page As on the HTML version, always use the Download button.

Here's where I heard about this:

How to prevent a PDF file for being corrupted . . . To protect PDF files from being damaged: 1.) never to open a PDF file in a web browser, 2.) download and save the PDF first, before viewing or printing it, 3.) compress the PDF files in a ZIP file, and then attach and send it through e-mail . . . https://www.datanumen.com/blogs/how-to-prevent-a-pdf-file-for-being-corrupted/

The Step-by-Step Guide to Fixing Corrupt PDF Files . . . http://www.technorms.com/44031/fix-corrupt-pdf-files

jscher2000 said

Hi bmakool, it's true that Firefox no longer runs PDF viewing plugins (since Firefox 52). If a site cannot let you see the PDF without a plugin -- just using the built-in viewer -- then you may not be able to view that site's PDFs in a tab. But I wouldn't expect to see that message on a lot of sites. Is it very widespread in your experience?

I cant open ANY PDFs in FF anymore. That's just the tip of the iceberg, FF has just become flat out broken since v 57. Mozilla is just totally doomed to bankruptcy IMHO.

sludge7051-x said

Here's where I heard about this: How to prevent a PDF file for being corrupted . . . To protect PDF files from being damaged: 1.) never to open a PDF file in a web browser, 2.) download and save the PDF first, before viewing or printing it, 3.) compress the PDF files in a ZIP file, and then attach and send it through e-mail . . . https://www.datanumen.com/blogs/how-to-prevent-a-pdf-file-for-being-corrupted/ The Step-by-Step Guide to Fixing Corrupt PDF Files . . . http://www.technorms.com/44031/fix-corrupt-pdf-files

Wow, none of that makes any sense. I have no idea what the heck your talking about.

boxfreindFF said

I cant open ANY PDFs in FF anymore. That's just the tip of the iceberg, FF has just become flat out broken since v 57.

If the solution in this question does not help, please start a new question with your system and add-on information. As you know, scroll down past suggestions to continue with the new question form:

https://support.mozilla.org/questions/new/desktop/fix-problems