If I may ask,

Are you using Firejail for Firefox?

If so, Firefox may be unable to access most files outside of its files and ~/Downloads

No, I am not using Firejail.

File not found Firefox can’t find the file at /tmp/pid-15374/etfExpectations_2022-04-22.pdf. Check the file name for capitalization or other typing errors. Check to see if the file was moved, renamed or deleted.

Looks like the file is not there. Post a screenshot showing the issue with file in that location.

As noted in the original post, the file can be loaded into Firefox directly by using File->Open. So the file is present in the directory and found by Firefox using this method.

Firefox can't load the file via drag-and-drop or by file association.

Here's a screenshot of the directory.

It has something to do with the file associations and files in the tempdir directory.

I have PDF files associated with Firefox. When I double-click them from, say, Thunderbird, the file is automatically loaded and displayed by Firefox. Of course, this isn't happening anymore, as described in my first post.

Instead of double-clicking a PDF file in an email this morning, I downloaded it to the designated download directory. Then I double-clicked the file icon, and Firefox opened the file without a problem.

Maybe it's an OS problem- I am running Ubuntu 21.10

Maybe check the permissions and owner of the file. I notice a lock icon on some of the files in the screenshot including the file you mention.

The locks are because of read-only, inability to read a file hides the file altogether from the user.

I am going to test a bit in order to see if I can recreate (obviously outside of my firejail)

I just found out that my firefox is not in a firejail, so thank you for that, but I can not reproduce.

The locks may actually be because of bad permissions, I tested it and even though ls -A did not show my file, nautilus magically was able to, even though the permissions were 600

Ok,

So I have found two separate errors.

File not found Firefox can’t find the file at {File that exists, but is blocked by firejail} Check the file name for capitalization or other typing errors. Check to see if the file was moved, renamed or deleted.

and

Access to the file was denied The file at {file that is accessible, but has 600 permissions} is not readable. It may have been removed, moved, or file permissions may be preventing access.

To be sure, which of these are what you have?

Oh--forgot to mention--I am using Ubuntu (Pop! because nvidia is pain) 22.04

I am not using Firejail.

File can be opened via File->Open, so I think permissions issue can be eliminated.

Issues with drag and drop can be a problem with crossing the sandbox.

Maybe, but I am not using a sandbox; that I know of.