I've looked at the code at the snopes.com web site. The javascript is some nasty code (but I'll spare everyone the gory details).
The script on the page loads and runs a second javascript from media dot fastclick dot net. I was able to capture a copy of that second javascript's code to examine for myself.
To get around Firefox's popup blocker, the second script writes an 'embed' tag into the page that loads a Flash file (from cdn dot fastclick dot net). The Flash file contains code in it to open the pop-under window. Firefox's popup blocker does not stop this window from opening, because the popup blocker is not looking at windows opened from Firefox plugins such as the Flash plugin.
A preference was added to Firefox to control windows opened by plugins, but this preference is not visible by default. To create this preference in Firefox, go to the "about:config" page (enter about:config in the address bar).
Right click in the window where the values are listed and select "New", then "Integer".
Enter "privacy.popups.disable_from_plugins" for the preference name (without the quotes), then click "OK".
Enter "2" for the preference value, then click "OK".
The new preference should take effect immediately - it should not be necessary to restart your browser before the preference will work.
The privacy.popups.disable_from_plugins preference can take one of four values:
0 = Allow Popups From Plugins
1 = Limit Popup Windows From Plugins (to dom.popup_maximum, even with popup blocker disabled)
2 = Block Popups From Plugins
3 = Block All Popups From Plugins (including white listed sites)
A good site to test the preference on after creating it is liquidgeneration dot com.
If you use the Adblock plugin for Firefox, it looks like it would be a good idea to block fastclick dot net. Users who never see pop-under windows may have ad-blocking software that is blocking fastclick dot net and other advertising domains that may be hosting this code, may not have the Flash plugin installed, may have javascript disabled, etc.
Links:
- Bugzilla bug
176079 - "popup blocking does not stop Flash from opening windows"
- Slashdot article: "
The Return Of The Pop-Up Ad"
Hope this helps...