Connected attics in 1880s frame rowhouses--should it be fixed? How?

I have a three-story row house built in the 1880s. Brick on the garden level, wood frame on the two floors above. The party wall only goes up to the ceiling of the top floor–i.e., our house’s attic is connected to the attics of the two other houses in the row!

This was most noticeably a problem when we smelled cigarette smoke wafting from the neighbor’s house. The problem was fixed by filling in the gap above a ceiling light fixture with insulating foam.

But we’re concerned that this could pose a more general hazard. Is it worth it to seal off the attics? If so, how to do this without breaking the bank (and the house)? The only access to the attic is through a 2″x2″ portal in the top floor ceiling, so we can’t exactly get panels of drywall or anything else that big up there without knocking down the ceiling.