Type this where you would normally put a URL:
about:preferences#content
You can block popups in general, but then (temporarily?) allow certain sites.
I block popups, run Adblock Plus and NoScript add-ons, and use an add-on call Video Download Helper to download videos I actually want to watch.
Point of order: about:preferences#content has been deprecated. It is now about:preferences#privacy.
For normal people, like everyone here except us, that'd be :
'alt' to reveal the Standard tool bar...
Poke 'Tools' => 'Options'. In the 'Options' dialog, in the sidebar, poke 'Security and Privacy'.
It is NoScript that kills the buggers. Once I 'Temporarily allow this page'. there they are.
I would imagine Ad Block could be set to custom block them, but that would be laborious unless they tend to come from only a few sites/vendors... never really looked into it.