All good things must come to an end, and the 1.5.x branch of Firefox is one of those good things. Today, the Mozilla team released a notification regarding Firefox 1.5. As Firefox 2.0 has been out for quite some time and is the only developed branch, eventually support for the older browser will be dropped. They are reminding the community that this is going to happen, and that users of the older browser should update. If you are still using 1.5, it's a good idea to update, particularly since the majority of addons created now are for 2.0 and most 1.5.x addons have already been rebuilt for compatibility.
The Mozilla team plans to stop supporting the 1.5 branch altogether next month.