Firefox can be set up not to store temporary internet files(under Tools > Options> Privacy > Private Data , tick off "always clear my private data when I close Firefox"). Also, cookies, the little operational helper-programs used by websites, can be dealt with in the same Privacy panel by setting up "Keep cookies until: [I close Firefox]"

One often never finds out just what's been done in a child pornography bust, and anyways, regardless of what's been done before, a new technique could always be used.

There was a case recently where an Australian who was a prominent member of an international BL site and also connected to one or two people locally who were doing illegal things (which he said he was not, himself), had his home broken into by police while he was away. Keyloggers were put on his computer and small cctv cameras in many parts of the house. This was under anti-terrorism legislation. Failing to come up with anything else incriminating, even after working over his YF, the police went through his computer. It turned out that in the past he had had some illegal images and had wiped them carefully at the Dept. of Defense standard over a year prior to these events. However, his new Windows Vista had crashed while opening one or two images, and had stored an undocumented type of error log including a small version of the crashed image.

This was sufficient to get him arrested and charged.

So one ought not to try to second guess these things.