Firefox 4 does not install

When dragging the Firefox icon to the Applications folder, I get:

"The operation can’t be completed because you don’t have permission to access some of the items."

Solution:
- Open a terminal
sudo -s
hdiutil mount /Users/jerome/Downloads/Firefox 4.0.1.dmg
cp -r /Volumes/Firefox /tmp
mv /tmp/Firefox.app /Applications

et voila