Request.InputStream returns empty stream in Firefox 3.5.11

Question

My ASP.net application allows users to upload files via AJAX. Generic Handler (.ashx) handles uploaded files. Up to now one user got error while uploading. User told that his browser is Firefox 3.5.11. I also setup that version and tested the system.

I realized that

context.Request.InputStream

returns empty stream. I couldnt find any solution for this.