It's stored on your PC in the window.localstorage for that extension. I used the Extension Gallery and Web Store Inspector to look at the code without installing the extension:
It might be easier just to save them as bookmarks. Right click on any tab in the tab bar and choose "Bookmark all tabs" or press CTRL+SHIFT+D. Use the Split Tabs extension to break them up into local groups first:
https://chrome.google.com/webstore/detail/extension-gallery-...
It might be easier just to save them as bookmarks. Right click on any tab in the tab bar and choose "Bookmark all tabs" or press CTRL+SHIFT+D. Use the Split Tabs extension to break them up into local groups first:
https://chrome.google.com/webstore/detail/tab-split/imjbfepo...
I've talked more about the benefits of using just bookmarks here:
http://news.ycombinator.com/item?id=5358109