Not allowing Firefox to install plugins from previous versions is kindof annoying. Especially when the developer doesn’t maintain something. The No-Referer plugin is very simple and handy, but hasn’t been updated to allow it to be installed in Firefox 3.5 or above.
In many cases, all that needs to be done is a version number needs to be incremented in the install.rdf file. I’ve posed a new version of this that works for FireFox 3.7 here