On my macbook, I had to completely remove Firefox in order to correctly get the contents of the ajax response using httpfox. This also involves deleting the firefox bij profile that deletes this Firefox folder (I could only find it through the terminal, not through the Finder):
/Users/<YOURUSER>/Library/Application Support/Firefox
Then install firefox again and install the httpfox add-on. I also tried a lot of cache settings by typing roughly: config in the firefox url, but not succeeding. Keep in mind that deleting your profile settings as I describe will also delete all your personal firefox settings. This concerned firefox 18.0.2 and httpfox 0.8.11.
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