I just checked my favorite add on for Gmail over at addons.mozilla.org and it is released under the Gpl v2.
The add on is Better Gmail2 by Gina Trapani of Lifehacker.com fame. She actually releases a bunch of add ons under the Better moniker https://addons.mozilla.org/en-US/firefox/addon/6076 (better gmail2) https://addons.mozilla.org/en-US/firefox/addon/6424 (better greader) https://addons.mozilla.org/en-US/firefox/addon/8010 (better flickr) https://addons.mozilla.org/en-US/firefox/addon/8032 (better amazon) https://addons.mozilla.org/en-US/firefox/addon/6460 (better youtube) groups.google.com/group/better-gmail-2-firefox-extension (Support forum for all better add ons) There is also another add on called better gcal but the Gpl v2 license isn't at the bottom of the page so I will message Gina on the support group and see if she can add that and get back to you. Regards Nathaniel -- http://gnuzilla.gnu.org
