> Hi,
>
> We want to store 64 bit integers in a FB 2.1 database (2.5 as soon as
> it's out and tested with our ECO app).
>
> What datatype should we use?

BIGINT, see http://www.firebirdsql.org/refdocs/langrefupd15-bigint.html

> Also, does FB have a special GUID datatype? (I suppose char would do,
> but it's really just a very large integer isn't it? 128 bit?)

Just use CHAR(16) with characterset OCTETS, you can use gen_uuid() to generate UUIDs (you have to wait for 2.5 for conversion from/to RFC4122 format.
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