Glass is a great mold to use as long as it's the right shape for the type of item you made. In your particular case, you made an item you want to remove from the mold. For future reference, a martini glass is the perfect shape for creating a cone-shaped product; a pair (7.5oz each) can be gotten at wally-world for a couple bucks. Whether the mold is glass, plastic or other, the shape must not have any edges or undulations that will provide the hardened resin a surface to grip against when you try to remove it. If properly lubricated, the hardened items will practically fall out of a cone shaped glass mold. Heavy-plastic funnels also make a good alternative.

In those instances when you pour an item with the intention of leaving it in the mold, shape is not so much a concern, but glass is still a good choice since it provides a good seal against long term exposure to water.

In the particular case of your first HHG, depending on what shape of glass you used, and whether or not you used a lubricant AKA mold-release-agent, you may or may not be able to remove the hardened HHG without breakign the mold. Putting it in a heated box, like an oven (or home-made box heated with a blow dryer) may allow it to cure some more, which means it will shrink a bit more (assuming you used traditional polyester resin like bondo) and perhaps coax it out of the mold, but, I strongly advise against using your regular kitchen oven as this will create some undesireable fumes that WILL be noticeable throughout your house and may linger for a good while afterwards in your oven - not very appetizing!

You can also put it in the freezer for several hours - this too may help an ornery item to become unstuck.

For future pours, yes, lubricate the surface of the mold before pouring. I've used various cooking oils - olive, and conaola, with good success, paint it on with a small brush.

Happy pouring.