From a Mac:

On the menu bar, click on the Firefox menu and select Preferences....
Select the Content panel.
Block pop-up windows: You can Uncheck this to disable the pop-up blocker altogether
from any website, or you can leave the box checked and click on Exceptions.

Exceptions: This is a list of sites that you want to allow to display pop-ups. To allow pop-
ups from the Lexington Aspen site, enter in the box

for address of website; click allow; then click Close.