Here's the link you need in relation to Mosaic's post:
https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/
However pop-ups are usually associated with malware, so I'd strongly recommend running a scan with Malwarebytes as well. Download the free version from here:
https://www.malwarebytes.org/antimalware/
Install it, taking care to deselect the option to install a free trial of the Premium version. Run a 'quick' scan but don't be surprised if it takes over half an hour to complete. If it reports that it's found malware, click the button to view the results, ensure that there's a tick alongside everything listed and then click the button to remove it all.