sp_BlitzFirst @AsOf Functionality Not Working?
Unfortunately, compatibility level 90 (SQL Server 2005) is not supported.

From the known issues list at: https://github.com/BrentOzarULTD/SQL-Server-First-Responder-Kit

"Only Microsoft-supported versions of SQL Server are supported here - sorry, 2005 and 2000. Some of these may work some of the time on 2005, but no promises, and don't file a support issue when they fail. (For example, we know the output tables won't work on SQL 2005 because one of the output fields is a DATETIMEOFFSET datatype, which isn't available in 2005.)"