Querying against FB 2.5 RC3 server I can see values 0, 1, 2 in the mentioned field. But release notes describe only 0 and 1. What does 2 mean?
I think it might be Idle after processing select, but before reaching end of data and releasing the statement as stated in the following link:
Enhance MON$STATEMENTS / MON$STATE to represent real CPU consumption
which was introduced in the 2.5 Beta 1 release as shown in Release Notes - Firebird Core - Version 2.5 Beta 1
Not quite sure if it was really added as the bug report suggested, but they also speak about a state 3 which would be Active during data retrieval. You could always ask in the Firebird mailing list which you can find at Firebird Lists and Newsgroups.