I installed Version 2.0RC9 Build 120817 on a shared coolhandle.com LAMP server and cannot do the initial admin login using Firefox v.15 I am able to log in as admin using IE, Chrome and Opera so this has to do with Firefox. After login using IE I am able to perform various user admin functions without trouble although I have not yet created a survey.
I also installed the latest stable release v 1.92+ build 120822 on the same account but in this case I am able to log in as admin and the software seems to function properly although I have not yet created a survey with it.
I found a few threads regarding Firefox problems but I think this is a new one. I do not have any problem with the vertical scrollbar that others reported earlier. The login failure manifests as a refresh of the login page without any message regarding login failure.
During the installation of RC9 the installer checked the server configuration and reported only one optional item missing, the PHP LDAP library. I doubt that this is the source of the trouble because other browsers work properly including IE.
The RC9 config file located in application/config has proper credentials for database access. Various entries in the _settings_global database table appear to be correct as best I can tell. The admin user record is correct and the password, although encrypted, must be correct because authentication succeeds in browsers other than Firefox.