Firefox does not see some files to upload that chrome does see. How can I fix

Some files that are visible in my PC's directory are not visible to Firefox to upload to attach to a Google g-mail, even though they are visible and uploadable to g-mail in Chrome. I created 4 similar (all pdf) files (printed from Adobe), but only 2 are visible to Firefox!!?? This has happened before. I try re-creating the invisible files, but have no success. Unless I can fix this issue, I'll have to stop using Firefox.