Open Firefox and click on the Tools menu and go down to 'Options...'.
Click on the Content tab.
Click on the Privacy tab.
Make sure the 'Accept cookies from sites' option is ticked and that the 'Keep until' option is set to 'they expire'
Note: If the 'Accept cookies from sites' checkbox is already ticked continue onto the next step.
Now that you have completed these steps click the OK button at the bottom window.
You should now close Firefox and then restart your PC.