I have uploaded a userscript that you can use to support FSFE when buying at Amazon. Automatic support for redirecting from libri.de to bookzilla.de is planned, aswell as any other affiliate programs FSFE might take part in.
In contrast to the AffiliateFox-Plugin, the userscript is compatible with non-Mozilla browsers. But it works on those, too, in case you prefer to install something where you can see the source, before installing it
For more information see http://blogs.fsfe.org/h2/userscripts/