I too was greeted by the update prompt but it did not proceed to update automatically.
I used to think that the update options were removed during packaging. I have seen that earlier versions of Firefox installed from the repositories had the Check for update option greyed out / removed. I did not check with FF 4 though.
Can anybody who still has 3.x installed from our repositories confirm this?
My thoughts were that since we were only installing / updating from repositories, this was done to prevent conflicts with the regular update process.
If possible, it would be better to set the Firefox update option to disabled and only allow add-on update checking while preparing the package. That would save a lot of such incidents for the already hassled packagers.