> Timberwolf's copyright is GPL or something similar.

Firefox is dual-licensed (L)GPL and MPL. For Timberwolf, MPL was chosen:

"The source code of the port will be made available only as far as the MPL requires it, i.e. all modified source code files will be available for interested parties, but new files will not. This is in accordance with the requirements of the Mozilla Public License."
http://www.amigabounty.net/index.php?function=viewproject&projectid=44

More info:
http://raw.githubusercontent.com/tfrieden/timberwolf/master/toolkit/content/license.html
http://en.wikipedia.org/wiki/Mozilla_Public_License

> a GPL protected code should remain open sourced after any modifications.

MPL is less strict in this regard.