I found it. The "storage.swf" was injected by the Firefox extension "pdfforge toolbar 5.0", installed by the open source "PDFCreator" from
http://www.pdfforge.org/pdfcreator/ - it also did install the Firefox extension "Widgi Toolbar Platform 5.0" which seems to be used to implement toolbars like the "pdfforge toolbar". If I wasn't using "FlashBock" Firefox extension, I probably would never have discovered this strange "storage.swf".
Thank you & have a good night.