Question: As the question says, where do each of the browsers store their offline data, be it cache, offline storage from html5, images, flash videos or anything web related that gets stored locally.

Answer: 
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`
I've just found that the flatpak chromium uses

```
~/.var/app/org.chromium.Chromium/cache

```

On Firefox I use the browser.cache.disk.parent\_directory "preference" in about:config to get it to use somewhere that's not backed up, to reduce backup sizes.