In Firefox, users should make sure that they do not have an add-on or extension that is blocking third-party cookies. There are many extensions / add-ons that may be blocking these. Here’s how you can view the extension or add-ons installed in your browser.
1.) Access the Firefox menu at the top right corner of your browser window and then click Add-ons.
2.) Go to the Extensions tab on the left side of the page. It will show the extensions currently installed in your browser.
3.) Note any extensions that may be blocking cookies (examples include Better Privacy and Cookie Monster extensions). You can check the functions of the extension by clicking on the Option button. Click on the Disable or Remove button for the extensions that block third-party cookies.
4.) Reload FarmVille 2 and check if the envelope is working.