Tech room descriptions lead us to believe that large bombs were launched at planets to bombard them by Terrans and Vasudans (Harbinger) while the Shivans used what appears to be a beam cannon (it might be assumed the Terrans and Vasudans use them to the same end by Freespace 2). Launching ordinance at a planet could conceivably be done in a few ways: Firepower (bombs and other weapons) launched from capital ships in orbit; bombs dropped by high atmosphere bombers; bombers actually launching bombs from high orbit; or some combination.
How would the beginnings of a planetary attack proceed in Freespace - keeping in mind the differences that might be incurred by the discovery of beam technology? For this discussion, it would be prudent to focus on how firepower would be directed at the planet and possibly how troops would then be landed, but not to delve into how ground combat would occur.
I'm tempted to say ships like cruisers and Orions hovering in low orbit launching bombs and blobs. It looks like utter devestation, but a good missile defense system could probably destroy a Harbinger while it was still in the very upper atmosphere, minimizing its damage. Blob turrets, if aimed at cities or military bases, could actually be effective if they have the range to attack a planet. Laser cannons from large capital ships would most likely do severe damage to an area over time.