Where do Chrome, Chromium and Firefox store offline data
Firefox store the offline data in ~/.mozilla directory, to be more specific in ~/.mozilla/profiles/xxxxxxxx.default directory.
Chrome uses ~/.cache/google-chrome directory for storing cache. Google chrome also uses ~/.config/google-chrome/Default directory for storing recent history, tabs and other things.
Like Chrome, Chromium uses as such with only the name changed. That is ~/.cache/chromium and ~/.config/chromium/Default