If someone tests Ubuntu Karmic Alpha 3, they will find that there is a new extension added called "Multisearch". This is what it does:
1)When you open a new tab, Firefox opens http://start.ubuntu.com/9.10 instead of a blank page
2)If you perform a search with the search box, instead of using the default Google search, it uses the Google custom search used by Ubuntu, which means that there is loss of functionality found in the default Google search look and feel.
3)You see on the top of the search results a big box containing ads.

The addon is also integrated inside Firefox, which means that you can't remove it with Apt or Synaptic completely from your system, while preserving Firefox. Also, it was probably added silently and without any notice, since according to the changelog entry:
firefox-3.0 (3.0.11+build2+nobinonly-0ubuntu2.me001) karmic; urgency=low

* add me001 multisearch feature for karmic alpha3
- add debian/extensions/*

However, it is nearly impossible for someone to guess what exactly this means.