it's not possible to access chrome://thumb and chrome://favicon from
extensions. There's a bug to add support for that which you can star:
http://crbug.com/11854
In general, you're going to get more useful answers to extension
questions on the chromium-extensions list:
https://groups.google.com/a/chromium.org/group/chromium-extensions/topics
Typing in "favicon" into the search box on that page found
https://groups.google.com/a/chromium.org/group/chromium-extensions/browse_thread/thread/ce373b2d04025ea
for example (though that thread is almost 2 years old, so maybe things
have changed since then).
Good luck,
Nico
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