On your Mozilla firefox browser search box, type in, greasemonkey. Click on the greasemonkey-add-ons for firefox.
A new site will launch click on Add to firefox.
add on tab will be launched and would ask you to install now. Click install now.
after the download finished. It will ask you to restart Firefox. Click Restart Firefox. 
restarting…. 
In your Add-ons tab in Tools---> you will see Greasemonkey installed. Also check on your Lower right that there is an icon of greasemonkey.
Congratulations!! you have successfully installed Greasemonkey Add-ons for firefox
