Thanks for your hints! Well, using 'mn ' instead is a nice workaround for the case you don't want to change neither keyword nor location bar entry.
Anyways, neither "Clear Location Bar" nor editing localstore.rdf did work. The solution was to remove the annoying URL from the history! So what does the "Clear Location Bar" button do at all? I don't know, at least it does not do what I expected. I wonder whether the reporter of http://bugzilla.mozilla.org/show_bug.cgi?id=117170 wanted to describe the same problem. The report is a bit unclear to me.
