>>>By default it's installed into C:\Program Files\Mozilla Firefox. BTW, Firefox 1.0 has been released today
>>
>>This directory is gone from my computer. Could it be that it was expiring by itself an auto removing because of the release date?
>
>I doubt that. Is it still in the Add/Remove program list?

As Sergey said, I doubt that was the case. I just upgraded to v1.0 using v1.0RC without any problems.