> So, are you saying that records that have been deleted, are not over
> written by new ones until a sweep is performed?
>

No, but a sweep does manually force the garbage collection to happen.
Deleted records that are no longer interesting to any active
transaction are cleaned up as they are discovered, or by a dedicated
garbage collection thread, depending on your version and server type.
After the garbage has been collected, the space the old records took
up can be reused by other transactions.

Now, to your problem.

Buy yourself a larger drive and move the database onto it. Unless your
transactions are poorly managed, databases tend to grow to a given
size quite quickly, then gradually increase proportionally to the
amount of new data. So even if you backup / restore (which will give
you a smaller database), within not too long it will probably grow to
a similar size.

35 GB is not too large these days (unless it is only 1 table of data
in which case you are going to hit an internal limit of Firebird).
Given the relative cheapness of 100 GB drives, you could buy a new
one, install it in the server and transfer the database to it.

Adam