The Mozilla Foundation is testing a patch to its Firefox browser that puts the kibosh on popup ads which have been slipping through the open-source browser's blocker.

In recent weeks, said the group, advertising networks, in particular Fastclick, have started exploiting the fact that Firefox's popup blocker doesn't stymie Flash and Java popups by default.

Firefox can disable such popups, but the current version has that feature turned off due to concerns that it would interfere with legitimate popup windows that some sites use. A manual method of changing the default to block plug-in popups was offered up by the foundation several weeks ago, but now it's testing an add-on patch that automates the chore.

The patch has been packaged as an extension to Firefox, and can be downloaded from here.

Unfortunately, it also blocks legit popups. The only way to access those is to "white list" the site from the built-in popup blocker. "This looks, at this stage, like a reasonable trade-off," said Asa Dotzler, the Mozilla developer who created the patch.