with the new knowledge we get from TBOB, we now know that Ford must have liked and participated in Halloween/Summerween—or at bare minimum Summerween.
We know this because TBOB has shown us that if Ford meets a paranormal entity who embodies a holiday and is policing whether or not everyone shows sufficient holiday spirit, he will not grudgingly play along. His first reaction is to get hopping mad and pick a fight with that entity over people's freedom to opt out of holidays.
Had the Summerween Trickster discovered Ford not showing sufficient holiday spirit and tried to chastise him over it, much less threatened him, Ford would've chased him off his property with a flamethrower jury rigged out of a lighter and a can of hair spray—if not outright hunted him down for the sake of any innocent children of Gravity Falls who just don't feel like wearing a costume. The Trickster would've either died in the 80s or been too terrified to ever show his cute little masked face at the Mystery Shack again.
Which means Ford must never have given the Summerween Trickster any reason to reveal that he punishes people who aren't Summerweening hard enough.
Which means when the Trickster arrived at Ford's door, he must've been met with enough holiday cheer to leave quietly and peacefully.
I wouldn't be surprised if Ford never decorated or dressed up; but I also wouldn't be surprised if, when the Trickster knocked on his door, Ford delightedly answered, "wow!! I didn't think I'd get any trick-or-treaters this far from town! Here, I've got a big bag of jelly beans, you can take the whole thing, I doubt any other kids are coming tonight!"
The Trickster is appeased.
He also might've mistaken the trench coat for some kind of gritty noir detective costume.
