Did some more testing on the SeaMonkey nightly build which caused me to do some bugfixing.
I increased the maxversion of SeaMonkey to 2.1a3pre
I solved a little bug: when using a new profile, the formhistory is not created yet (by SM) and the extension should show a warning about that fact. Instead it displayed an error about the directory not being writable instead of a warning about the database not being created yet.
Added a workaround for SeaMonkey: In order to display translators/contributors in the about-dialog, the new Addon object (obtained from the new Addon manager) has no getResourceURL() method where it does exist(!) in FF.