Problem
If you follow the instructions http://greasemonkey.mozdev.org/source.html you will not get the correct files that you can build a Firefox XPI from. [Summary: anonymous login to cvs and download the HEAD branch]
To contrast, the compiled XPI has .xpt files in the content/ directory. So there is definitely a build step which is not simply zipping the src directory and renaming it to .xpi.
Solution
From the debian guys' search for the same solution comes the hint.
Set CVS to download the v0_6 branch. This comes with a build.sh script in greasemonkey/src . I will report on the build steps later.

