The J-Walk Blog: Stopping Ad Pop-Unders With Firefox

"To block pop-ups from plugins, open your Firefox 1.0 or 1.0.1 browser, type about:config in the address field. Right-click in the resulting config page somewhere and select New -> Integer. Type privacy.popups.disable_from_plugins in the resulting dialog, hit OK, type 2 in the next dialog and you're all set."

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Works like a charm!
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