Yes, the built-in PDF should still be working. Let's try a refresh:

The Refresh feature (called "Reset" in older Firefox versions) can fix many issues by restoring Firefox to its factory default state while saving your bookmarks, history, passwords, cookies, and other essential information.

Note: When you use this feature, you will lose any extensions, toolbar customizations, and some preferences. See the Refresh Firefox - reset add-ons and settings article for more information.

To Refresh Firefox:

1. Open the Troubleshooting Information page using one of these methods:
   • Click the menu button , click help and select Troubleshooting Information. A new tab containing your troubleshooting information should open.
   • If you're unable to access the Help menu, type about:support in your address bar to bring up the Troubleshooting Information page.
2. At the top right corner of the page, you should see a button that says "Refresh Firefox" ("Reset Firefox" in older Firefox versions). Click on it.
3. Firefox will close. After the refresh process is completed, Firefox will show a window with the information that is imported.
4. Click Finish and Firefox will reopen.

Did this fix the problem? Please report back to us!

Thank you.

Did a refresh - Adblocker went away etc. etc.

When I click on the Download DOWN arrow - Show all downloads - I get a list.

Double-clicking on most of the items of the list produces no response at all.

For one or two I get the '~ not a Win32 application' message noted earlier.

User987654321 said

Attempting to read a downloaded pdf, i get "blah.pdf is not a valid Win32 application"

Does this happen with all PDF files, even when you right-click and save a PDF file to your computer and then try to open it?

The KB article Use Adobe Reader to view PDF files in Firefox has a link to this sample PDF file you can use for testing:

• http://static.mozilla.com/moco/en-US/pdf/mozilla_privacypolicy.pdf

Two other thoughts:

(1) Could you set Windows to display all file extensions to make sure there isn't anything after .pdf which could be making the file an executable?

https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

(2) Can you view the downloaded PDF if you use either:

• drag-and-drop the PDF on an open Firefox tab
• use "Open File" (Ctrl+O) to open the PDF into a Firefox tab

It is also possible that the Windows Registry key for .pdf is corrupted and point to an invalid application.

• HKEY_CLASSES_ROOT\.pdf