De Eu continuo recebendo esta mensagem" seu computador deve ser reiniciado para concluir uma desinstalação anterior do firefox "?
rename file C:\Program Files\Mozilla Firefox\firefox.exe.moz-delete to firefox.exe.moz-delete.old and the installer will work, if you rename or remove the complete folder you will also loose all your settings and plugins
I had to rename the file called C:\Program Files\Mozilla Firefox\firefox.exe.moz-upgrade to firefox.exe.moz-upgrade.old for it to work, but it finally worked.