If you are installing Greasemonkey on a Mac, the Greasemonkey extension is saved as an XPI file. Within Firefox, choose File/Open and select the XPI file you saved.
After downloading Greasemonkey, your computer should come up with the standard "you are about to install an extension" dialog and allow you to install after a few seconds.
Close all Firefox windows and restart Firefox.
After re-starting, you should see a small smiling face of a monkey in the lower right corner Firefox. That means Greasemonkey is installed correctly
While the file is displayed, go to Tools. Select "Install User Script". The script is already configured, so just click OK. Click OK when prompted to refresh the page.
Click the 'Reload' Button in Firefox to activate the editor.