Strange Update on u3 firefox

I downloaded the u3 firefox and let it apply patches and used it for two days.
Just a few minutes ago, it announced that it had downloaded the 1.5 version and wanted to update. I let it do so and then immediately fired it up and did the check update. Then it went through the update process to 1.5.0.6 What is going on?

P.S. I prefer regular portable applications but I feel constrained to use a U3 drives because of the issues of compatability.