I have noticed that when I set mail retrieval to ETRN to set a schedule that after I hit save I get a blank page on the right hand side. I then select email on the left hand side and I get the right hand page back. When retrieval is reset to SMTP I do not lose the right hand side.
I don't know if this happenned before installing your fetchmail contrib or after as I had to need to alter the retrieval mode before.
It may also be a firefox issue .. will have to investigate further.
Regards,
William.