Mozilla Firefox disk cache
Mozilla Firefox uses disk cache to store resources fetched from the web so that they can be accessed quickly at a latter time if needed.
There are 2 known disk cache formats:
Mozilla Firefox disk cache format version 1
Mozilla Firefox disk cache format version 2
Firefox 1 to 31
Mozilla Firefox 1 to 31 use the Mozilla Firefox disk cache format version 1.
On Linux Mozilla Firefox 1 to 20
/home/$USER/.mozilla/firefox/$PROFILE.default/Cache/
On Linux Mozilla Firefox 21 to 31
/home/$USER/.cache/mozilla/firefox/$PROFILE.default/Cache/
On Mac OS
/Users/$USER/Library/Caches/Firefox/Profiles/$PROFILE.default/Cache/
On Windows XP
C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\Mozilla\Firefox\Profiles\%PROFILE%.default\Cache\
On Windows Vista, 7
C:\Users\%USERNAME%\AppData\Local\Mozilla\Firefox\Profiles\%PROFILE%.default\Cache\
Firefox 32 and later
Mozilla Firefox 32 and later use the Mozilla Firefox disk cache format version 2.
On Linux
/home/$USER/.mozilla/firefox/$PROFILE.default/cache2/
On Mac OS
/Users/$USER/Library/Caches/Firefox/Profiles/$PROFILE.default/cache2/
On Windows XP
C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\Mozilla\Firefox\Profiles\%PROFILE%.default\cache2\
On Windows Vista, 7
C:\Users\%USERNAME%\AppData\Local\Mozilla\Firefox\Profiles\%PROFILE%.default\cache2\
References
Example Mozilla Firefox Cache files, by firefox-specimens project