In the download dialog for a rar file there should be an option labeled "Do this automatically for files like this from now on."
Just click "Save to Disk" and click the above check box and click "save file". It will be remembered for all future rar downloads.
From a quick search I found the page at http://kb.mozillazine.org/File.....ad_actions for more on the check box.
It appears there is a bug that makes that box not work right in Firefox 2.0
It may be fixed in Firefox 3.0, I'm not sure as I haven't tried it.