If you've upgraded your web browser to Firefox 1.5, and you had the AdBlock extension installed, you may have noticed some problems. The Adblock team has released a new version which you must manually install from the Mozilla Update web site:
https://addons.mozilla.org/extensions/moreinfo.php?application=firefox&id=10
It also fixes some other problems, like the incompatibility with Flash 8. Happy surfing!