I think i may have a reason why and thats because we had a security mod installed that filter high payloads.
I have a tech looking at this.
However, times of the day precicesly may help me look in the error logs and see what error (if any)
please make a note of the time and I will look into this. Also if you know you IP this helps but dont worry.. I will trace it and see if it is the same error.
Firefox must handle this stuff differently.