This particular version of firefox gets an error "the file type you are attempting to load is not valid" when trying to upload a pdf file. All other file types work. I read on codeigniter forums and found that there could be an issue with what the FF is sending to CI in the headers and that I can edit the php line in the mimes.php in the config folder.
However, the suggested change has already been applied to my mimes.php file so I was thinking there is another exception that needs to be added.
But I do not know how to read the headers being sent to CI to know what else I might add.