0. Download archive from here.
1. $ tar -zxvf thunderbird_to_status_bar_v2.1.tar.gz
2. $ mv thunderbird.status@gnome-shell.rdiachenko.org ~/.local/share/gnome-shell/extensions
3. Type 'r' in Alt+F2 prompt to restart Gnome shell
4. Install gnome-tweak-tool:
$ su -c "yum install gnome-tweak-tool"
5. Press Alt+F2 and type in 'gnome-tweak-tool'
6. Choose 'Shell Extensions' and switch on 'Thunderbird Status Icon Extension':
7. Type 'r' in Alt+F2 prompt to restart Gnome shell
More useful extensions here: http://roman-ivanov.blogspot.com/2011/11/fedora-16-useful-extensions-for-gnome.html