What have you done so far?
New fuel filter?
Winter (#1) diesel?
Or a blend?
Or an additive?
Because regular #2 gels below 25 degrees.
What oil are you using, a winter oil or a synthetic?
Do the glow plugs (or Ford system) work & are you using it?
Do you have a good strong battery?
A block, tank, or hose heater really is needed for these old girls if it gets below 20 & you let them set more than 2-3 days between running.
So, all of the above should give you a good running winter diesel tractor. :)
Ether is needed once in a while, but can be real dangerous when used with manifold glow plugs (Ford - blow up manifold) and are just not good for the engine as others have said.