Go to Permissions in the ‘Privacy & Security’ section of the ‘Options’ menu. Either via the Firefox menu button (3 lines in the top right corner) or by clicking on the ‘Padlock’ icon in the address bar and then clicking the ‘Cog’ icon next to permissions.
The following options will be displayed:
Next to “Block pop-up windows” click the button “Exceptions…”.  In the Dialog that opens, add “poker.goodgamestudios.com” (without the quotes) and click “Allow”. 

Click “Apply changes” to add the exception.

Please note: If you are using a different version of Firefox you may see the option “allow pop-ups for this site”, please select this option instead.