Is it worth replacing a door gasket on a 25 year old freezer. It is a Kenmore 6. Upright. It does not seal tight anymore.

Try sealing the door with tape to see if the thermostat will work to turn it off.
My Kenmore upright freezer had a separate cross seal down quite low, on the freezer not the door. I reversed the door seal so the damaged top part was below the cross seal and voila, no more leak.
If it needs a thermostat I suggest getting it sealed first. Mine was only 10 or so years old but Sears Parts said seal no longer available. Good luck

If it is a push fit gasket it simply pulls out and the new one pushes in to the groove. Warm in hot water before fitting to ease it in. If the seal is part of the door, you have to replace the door as a complete assembly. Rare models have doors that can be taken apart and reassembled with a new gasket fitted but i don't think Kenmore is one of those.

symptoms you are describing are faulty door seals, try putting a thin piece of paper between door and frame to make sure doors are sealing,also when did you do a full manual defrost??(once a year even with auto defrost)

That could be one of two things, one the door is not aligned properly and more important the gasket has failed and needs replacement. The snow as you call it is ice condensed from the atmospheric air that is getting by the gasket. You may want to check the door gasket and sealing surfaces especially in the area of the "snow".
Good luck,
Gilshultz

its a manual defrost freezer, it is normal to ice up, if it is unusually more than you have noticed over the past 40 years, see if the door liner is cracked around the screws under the gasket, if so put some washers there to reseal the door liner

The first thing to check is door gasket< bad corners,gap in side,bottom or top. Close door on a dollar bill and try to slide it top to bottom side to side and if it slides fairly easy then seal is not doing good. If you find dirty area clean it off along with the cabinet side as well.If you find the side is leaking then heat gasket with hair dryer and apply vasoline to gasket to help it move freely. If you find tear or hole then replace it.

The door seal is at fault. Regardless of the small tear it is not sealing well.
Take a dollar bill and close it in the door. Then pull on the bill to feel resistance.
You will find it pulls easily in some place. That is where your air leak is that is causing the condensation.
Do this about every 6 inches all the way around the seal.

You can try replacing the seals and or adjusting the hinges.
That may work.

Be aware that if the unit is over 10 years old it is probably using twice as much power as a new one would and costing you an extra $75 to $100 annually to run over a new unit. Could be more than that if the door seals are not tight as well.

if the gasket isn't torn and the gap isn't caused by a door hinge problem you might try putting some vaseline on the door seal. If that doesn't help you may have to replace the seal.

You may need the hinge adjusted or a new door gasket but you might first try putting a layer of vaseline on the gasket. If none of those things help the problem is probably other than the door seal.