How to stop popups from open on websites in Mozilla FireFox
If Firefox has blocked a pop-up the icon appears in the address bar. A message will also appear at the top of the window like this Click the options button to see a choice of actions you can take.

To manually set or unset popup blocking follow these steps:
1) Open Mozilla FireFox
1) Click the Mozilla FireFox button at the top left of the window, and click options

1) Click the content button on the top row of icons

1) You can click / unclick the 'Block pop-up windows' tick box to enable or disable pop-up windows
2) Click OK to save the settings