If you use Vista (all editions), you may not be able to install plugins and it will be noted as "failed". This is due to the Vista's User Account Control disallows installation of programs without administrative rights.
To fix this, run Firefox as an administrator, install the plug-in, close Firefox and run as normal again. Remember to close and reopen firefox to make sense out of User Account Control.
User account control info: http://technet.microsoft.com/en-us/windowsvista/aa906021.aspx