Firefox won't open PDF in Preview

Hello,

I'm having a problem with opening PDF's. When using Firefox and attempting to open a PDF (note, not all PDFs - only some), I select to open with Preview but the document opens up in the Text Editor. It appears to be losing the extension, because I can go in and add an extension of .pdf and then open the document in Preview fine. If I am using Safari and click on the same document, it will open in Preview without any issue.

Does anyone know what would be causing this issue while using Firefox & how to remedy it? Thanks in advance for any assistance & please let me know if any further specific information is needed.