I will here assume you refer to these add-ons:
https://addons.mozilla.org/en-US/firefox/addon/classicthemerestorer
https://addons.mozilla.org/en-US/thunderbird/addon/zoom-button-for-thunderbird/
https://addons.mozilla.org/en-US/thunderbird/addon/manually-sort-folders/
https://addons.mozilla.org/en-US/firefox/addon/facebook-disconnect
https://addons.mozilla.org/en-US/firefox/addon/gdc
https://addons.mozilla.org/en-US/firefox/addon/twdc
If you look at their licenses (written after "Released under"), they
respectively are:
Mozilla Public License, version 2.0
Mozilla Public License Version 1.1
GNU General Public License, version 2.0
Mozilla Public License, version 2.0
GNU General Public License, version 3.0
GNU General Public License, version 3.0
If you search them in https://www.fsf.org/licensing/licenses/ you will
discover they all are free software licenses. You can access their source
codes (and study it) following links from the respective "Add-on home pages".
