If you want to stay within code, you must use an intumescent sealer.
Home Depot, Lowes, and Menards sells it in their caulk section. It's a
caulk that expands with heat and stops the spread of flames. Never use
ordinary foam.

If the fixture isn't approved for direct contact with insulation, you
can build a dam around it with sheet metal duct.

We just removed a dropped ceiling in our kitchen and put four 6"
recessed lights in the ceiling. There appears to be a slight area
around the base of the lights that's not sealed tight. We were
wondering if there is something acceptable to seal around these light
in the attic to prevent AC leakage? Foil ducting tape? Just put
insulation around them? Spray foam sealant (we're hessitant on this
as it states it's still combustable after cured)?

And should we leave the domes in the attic free of insulation so the
heat from the lights doesn't tranfer into any insulation that might be
over the light fixtures?

Thanks!