I cannot install firefox version 3.5.7 from LiberKey

Hello guys,
I cannot install Firefox version 3.5.7 from LiberKey. I have attached a screenshot of the problem below and I need help because Firefox is the browser that suits my style of browsing.
Thanks in advance.

LiberKey Inst v1.2.63
An error occurs while trying to rename the application folder.
Check that the app is not running and that no file are open in the application folder.
Retry Cancel
