You can't.<br /> Firefox keeps a dirty flag (dirty bit 0x0f: 0x01) in the _CACHE_MAP_ file and clears the cache when that bit is found set indicating that Firefox didn't close properly. *http://mxr.mozilla.org/mozilla-central/source/netwerk/cache/nsDiskCacheMap.h *http://mxr.mozilla.org/mozilla-central/source/netwerk/cache/nsDiskCacheMap.cpp ---- Is Firefox crashing often? See: *http://kb.mozillazine.org/Firefox_crashes *https://support.mozilla.org/kb/Firefox+crashes If you have submitted crash reports then please post the IDs of one or more crash reports that have this format: *bp-xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx You can find the IDs of the submitted crash reports on the <i>about:crashes</i> page. *You can open the <b>about:crashes</b> page via the location bar, like you open a website. See: *http://kb.mozillazine.org/Mozilla_Crash_Reporter *https://support.mozilla.org/kb/Mozilla+Crash+Reporter If you can't open Firefox then see: *http://kb.mozillazine.org/Mozilla_Crash_Reporter#Location_of_crash_reports *http://kb.mozillazine.org/Mozilla_Crash_Reporter#Viewing_crash_reports *https://developer.mozilla.org/en/How_to_get_a_stacktrace_for_a_bug_report