Why can't I delete Firefox from my apps folder?

I downloaded the new version, and when I drag it into my apps folder it asks me if I want to replace it. I say yes and then another dialogue box comes up and says I don’t have sufficient privileges (which I do).

Now I’m just going to delete the old one from my apps folder and put the updated version in my apps folder. When I try to move it to the trash, it says one or more items cannot be found. What should I do?