> Logically it is correct behaviour: but WHERE <value> = [constant],

while accepted as syntax, does not make much sense as a predicate.

Not only it makes sense but it's often used to test optional filters
like in "where :filter is null or column = :filter".

SQL-wise, <value> = [constant] is equivalent to [constant] = <value>.
In fact, it's only an implementation detail of firebird to behave
differently in each case, and this shows in the "for select"
construct.