Have you tried contacting your systems administrator or trying again from an account that has permission to install software on the computer?

You can try to repair the Firefox permissions, for details see http://support.apple.com/kb/HT2963?viewlocale=en_US

An alternative approach is to download the latest version of Firefox from http://www.mozilla.com/, then trash the current Firefox application and install the latest version. The user data (bookmarks, passwords etc) are stored in the profile folder, trashing the application will not remove the profile folder, and will be used when you re-install Firefox.

Thanks TonyE for your reply, I will try it.