From the Mozilla Developers Blog:

On Wednesday, May 30, we expect to begin distributing Firefox 1.5.0.12 and Firefox 2.0.0.4 via automatic updates. These are standard stability and security updates for the browser to ensure a fast and secure online experience for our users. We expect this is the final stability and security release for the version 1.5 product series.

Now, for those of you still on the 1.5 product series, you will be happy to know in a few weeks you will be offered an upgrade to migrate to the current 2.0 product series (should be 2.0.0.4) via the Major Update mechanism.