You need to save .xpi locally, open it up with (win)Rar, extract install.rdf and edit it with notepad or another text editor. Change the correct line to this: 1.5+ save it and add the modified version to .xpi file. You should now be able to install it.
Get Ver 0.2.4 in author’s homepage, here is the link: http://superdragandgo.mozdev.org/installation.html
