Problem in uploading i.tax return

When I click on digitally signing the return either by .pfx file or through usb token, I get an error message of file not being found. The fake path is not being detected and hence I am unable to digitally sign the return.
I am using Mozilla Firefox for this purpose since the income tax website does not open using explorer on my PC.
I have enabled java scriptt and also clicked on the option to load image automatically.
Please suggest what can be done.