What happens when you try to open a PDF -- error message, blank tab, download dialog, nothing at all?

You might switch around among the options before landing back on "Open in Firefox" to see whether that cleans up the setting: View PDF files in Firefox or choose another viewer.

When I check my banking site everything works as normal but when trying to download any statement (eg. credit card or savings account), the page logs out and I have to log in again. I've tried what's in the article by changing pdf viewers and starting in safe mode and even refreshing. Nothing has worked. I tried other sites also to try to open or download a pdf. I tried to open a pdf on how to repair a garage door lock for example and various other pdf's for instructions on other things. When I try to open any pdf it doesn't open the first time - instead Adobe Acrobat displays how this file type isn't compatible. Then I press the to open the pdf again and it opens as normal. After to FF updates, my pdf viewer was automatically set to OPEN IN FIREFOX and without changing that setting, everything still worked. I changed the setting to ACROBAT READER and it also worked. Then there was another FF update and after that some pdf's open and others don't. I figure it's a compatibility issue between FF and ACROBAT. Remembering that pdf's open when I click on them the second time. As for my banking, I don't get a chance to click a second time as it logs out and have to start again. The bank says it's an issue with FF or possibly there security certificate. I believe it must have something to do with conflicts between the pdf viewers and FF got it wrong. I have the latest Windows and up to date on everything else. Is the problem with Windows or FF? It's not my bank as the issues only happens with pdf's and my banking statements are pdf's. FF seems to have turned this problem into our problem when it must be a FF problem. I've never had any problems before with FF. The problems started when they introduced the built in PDF viewer. Either get rid of it or repair it and make it compatible as before.

The built-in PDF viewer is many years old now, so I don't think the fact that the option changed name from "Preview in Firefox" to "Open in Firefox" is the problem.

Here is what I suspect is happening: the download is failing the first time, so Acrobat only gets a partial file or an error message file, which is not a valid PDF. However, the download works the second time. If you keep an eye on the Downloads list in the case that the PDF can't be opened, and note the file size listed, when you download again, is the working file much larger? If so, that would support the failed download theory.

However, I don't know why Firefox would bother feeding a failed download to Acrobat, so maybe it's something a bit different. Here's what I suggest to investigate.

First, if you have never changed this setting, please set Windows to show all file extensions so that we can rename file extensions effectively: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

The next time you see a failed download, could you try this:

• On the Downloads list, click the icon at the right side to "Open Containing Folder" -- Firefox should launch Windows' File Explorer with the download highlighted
• right-click > Rename on the file, and change the extension from .pdf to .html
• Then double-click the file to open it in Firefox to see whether it is actually a web page showing an error message -- if so, what's the message?