Index X is corrupt (missing entries) in table XYZ (144)

This means that there is a problem with X-th index for table XYZ - there are records that exist in table XYZ, but aren't present in index. You should first try to rebuild the index. Find its name in RDB$INDICES and then run

ALTER INDEX <index name> INACTIVE;
ALTER INDEX <index name> ACTIVE;

In fact, in current Firebird versions it is enough just to make them active:

ALTER INDEX <index name> ACTIVE;

If this happens often, or you cannot rebuild the index, make sure your database has forced writes on and check for possibility of faulty RAM module, hard disk corruption or suspend/hibernate corrupting the filesystem cache for the database file. You can also use IBFirstAid application by IBSurgeon to get more diagnostics.