Scenario
When attempting to install or uninstall Forefront Identity Manager 2010 R2 (FIM) you receive the following error:
Textually:
To install FIM portal, the setup needs to run under SharePoint Farm administrator account with at least Open permission that allows users to open a Web site, list, or folder in order to access items inside that container. Please make sure you are a SharePoint Farm administrator with Open permission and then click “Retry”. Click “Cancel” to abort setup.
Issue
When you check the farm administrators group you are a member. What you are not is a Site Collection Administrator. The Site Collection Administrators are individual users – just two of them – not groups! Here’s mine:
But I’m running the installation as *the* administrator account (because you need that to uninstall FIM SYNC from control panel!):
As soon as I add myself to the Primary or Secondary Site Collection Administrator (Central Administration | Application Management | Site Collections | Change site collection administrators) the process completes as expected.
Summary
The principal you use to uninstall FIM should be the installation account. 
Being serious again…Specifically, to install or uninstall FIM Portal, you need to be a local administrator on the server (a domain user that is a direct or indirect member of builtin\administrators); a member of the SharePoint farm administrators group; and you need to be defined as either the primary or secondary site collection administrator.
References
