Getting the favicon for tab is done externally

`tabs.favicon` is deprecated as of 1.15 and `places/favicon`'s `getFavicon()` does not work in many cases, as it is still marked unstable. Therefore, for now, the getting of tabs' favicons is done using Google. If a better way is found or the above method becomes stable again, the add-on will get updated accordingly.

No repository file, test, or entry point is named. Start by locating the code that retrieves tab favicons, then compare the deprecated tabs.favicon and unstable places/favicon getFavicon() APIs with the current Google-based approach. Done means a reliable in-add-on favicon retrieval path is identified and adopted if available.

Written by the indexing model from the issue text.