Hello Hellriser, and welcome to Seven Forums.
As a test, extract the files to a folder on your Desktop instead. Next run the setup.exe file from within the extracted files to see if it will install now.
You should be able to just run the Firefox Setup 4.0 Beta 12.exe
to start installing it. No need for you to extract anything from it.
Right click on it, and click on Run as administrator
to see if that may allow it to install afterwards.
Hope this helps,