First you have to modify install.rdf file. Download the extension without installing it and open with 7zip or other. Open install.rdf with notepad and find the following line. <em:maxVersion>2.0.0.*</em:maxVersion>
Change the it to <em:maxVersion>3.0.0.*</em:maxVersion> to make it compatible with FF 3.0 and above. Save the file and install extension by draging it to the addons window in Firefox. You may have to extract the xpi file (extension file), edit the rdf file and then drag it into the original xpi or maybe recompress into an xpi file.
