Workaround for FF v3.xx Rated 5 out of 5 stars

Download file signature-0.4.0.4.200610221528-fx+tb.xpi with "Save link as" to your local harddisk. Then edit file install.rdf within the .xpi archive (.xpi=.zip) using a text editor like gedit. Find firefox targetApplication section and change 2.0.0.* to 3.5.0.*. Save file and update archive. Then use the firefox File/Open file dialog to install the extension. It will work without any problems. Enjoy.

Have a nice day
rfinder

This review is for a previous version of the add-on (0.4.0.5.20070419).