Ajax upload: Firefox data resubmission.

My site was uploading files perfectly until recently but I'm presuming that there was a security fix in the meantime which has stopped it working.
My users need to upload drawing files, but these are now blocked with a message:
"To display this page, Firefox must send information that will repeat any action (such as a search or order confirmation) that was performed earlier."
Firefox then cancels the upload.
Google searches tell me that this is due to CodeIgniter using redirects.
Has anyone else had this problem?

~ Dave