The purpose? To encourage additional and improved add-ons for Firefox, of course.
It's similar in many ways to Atlassian's bounty program, which is giving away six $5,000 bounties for individual plug-ins built for Jira, Confluence, and its other software.
Bounty programs have been around for years. The Ximian team used these somewhat effectively early on at Novell (and prior to that), which was my first experience with them. Since then, the number of bounties has grown considerably within the open-source world.
I personally am not a big fan of bounties because I don't think they go to the heart of why many developers write open-source code in the first place: pride of ownership, experimentation, intellectual pursuit.
But, if nothing else, they do call attention to a need: more plug-ins for Firefox and the Atlassian projects, in these cases.
It's not as if Mozilla is hurting for Firefox plug-ins. But it may be that it's trying to remind developers to update their Firefox 2 plug-ins for Firefox 3, and this offers a convenient, relatively inexpensive way to do so.