You only need one version, but make sure that there are two versions installed. Sometimes the Windows control panel can list 2 versions of Firefox even if only one version is installed.

If you installed the Firefox beta and then updated it to the release version you will likely have 2 versions installed as the beta would by default install in a different location than the release version.

Thanks for the info because I was going to delete the 3.6. one. I have the regular 4.0, not the beta, so it is in my control panel list of programs with the other one. My new browser is working just fine, so "if it ain't broke, don't fix it" applies here. I am going to leave things as they are.