Firefox is currently caching IFRAMEs. This means that if a site is designed with dynamic tagging in the IFRAME, the IFRAME contents are not being refreshed with the new dynamically generated tags, etc, on page reload. This bug has been found and logged with Mozilla, the organization responsible for Firefox, but it is not slated to be fixed as of yet. This should not be affecting ad impression counting, however, it may affect things like changing sizes or targeting parameters because these will not update on the page reload.
Details on the logged bug can be found here: http://bugzilla.mozilla.org/show_bug.cgi?id=279048
