The Firestone airbags, or any airbags are like helper springs, and the Sumo Maxime springs (I have them installed) are similar in their action. They do nothing (or hardly anything) to soften the ride, because they are in addition to the original springs.
There are pretty detailed discussions about air bags as helper springs over on iRV2, and some engineering people explain why air bags can't improve the ride by any measurable margin.
If one would want to get any benefit from those airbags in relation to a softer ride, one would have to change the original leaf springs for softer ones (Kelderman is doing this), and I am looking into this to do this in combination with my Sumo springs.
I found that all Ford F series springs would fit the F53 chassis, and that the spring ratings step down by 600 lb each. That means, the springs of the F450 carry 600 lb less than the springs of the F550/F53, and the ones of the F350 carry 1200 lb less, etc.
In my case, I probably would need to install the F450 springs to get a softer ride and have the Sumo springs handle part of the job.
Another way to soften the ride is to find out he exact weight of the rig at the front axle, and subtract that weight from the load for the springs. Once one knows the difference, one can load down the front to get to the weight of the spring rating (I guesstimate that my front needs about 400 lb more weight). To test this out, I plan to add steel plates to the front (ahead of the front wheels), after I found the exact required weight by going to a truck scale.
Once weighted down to the proper weight, I will take my motorhome on a long trip and see how this does. That experience I will use to change the springs in the front to compliment my Sumo springs, and hopefully achieve a ride quality that is similar to an air ride.