Upon checking the extension with FF 4.0b2pre I discovered that the XML-export was broken, it produced an empty file. It turned out that obtaining the name and version of the extension, which is added to the XML-file, used some obsolete methods on the rdfExtension handler I had changed recently to support the new addon-manager.
Having fixed this bug I noticed an exception in the errorconsole which was caused by the fact that on a fresh profile the cleanup db not yet exists. If the preferences dialog is openend, it tries to determine the filesize which will cause an exception to be thrown.
Having fixed these issues I increased the maxVersion for the extension on Firefox to 4.0b2pre.