In February, Mozilla made it known that plans were underway to implement ad support in Firefox, arguably the organisation's most prominent piece of software. A posting on Firefox's bug-tracker shows a complete list of said sponsors and includes the likes of Facebook, Amazon and Wired.
The post itself is from April 29 and is apparently definitive, except for the final review process of artwork of a couple of sponsors:
1. Facebook 2. YouTube 3. Wired 4. Lightbeam 5. MoFo 6. Wikipedia 7. Trulia 8. Amazon 9. Webmaker
According to gHacks Martin Brinkmann, the tiles are live right now in the latest Nightly build of Firefox. Unfortunately (or perhaps fortunately) I wasn't able to get the tiles appearing — I just saw a blank set of boxes. This may have to do with being in Australia, but we'll see what happens once the "feature" makes its way into the mainline.
If you'd like to see what they look like, you can check out a screenshot here.
Update directoryLinks to have actual links and images [Bugzilla, via gHacks]