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.
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