Smart Bookmarks requires the Java 5.0 plugin for Firefox.

MS Windows:

Installing the latest version of Java from Sun should install the plugin for Firefox automatically.
If that fails, try these instructions provided by Mozilla.

Mac:

1. Go to http://www.apple.com/support/downloads/java2se50release1.html to get Java 5.0 for Mac.
2. Install the Java browser plugin available at http://javaplugin.sourceforge.net/.
3. Once you have installed the items listed above, open the Applications folder on your Mac and go to Utilities->Java->J2SE 5.0->Java Preferences. Under the General tab, select J2SE 5.0 from the Use Version: dropdown and click Save.

Linux:

First install the latest version of Java from Sun's web site.
Then follow Mozilla's directions for installing the Java plugin on Linux.
This involves creating a symlink, often in your ~/.mozilla/plugins directory.