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Or are they liable to explode and perhaps break the glass or start a fire.
I have got loads of them, and don't want to put them in landfill.

They don't seem to compost, remaining largely unaltered after years in a compost pile.

You could try a few in an outside campfire as a test.Years ago a bucketful of butternuts (similar to walnuts)in my fathers wood stove put on quite a show.

Quite a few food processing factories run on the heat from burning "stones" that they remove from fruit so as, VT says, try one and see! At your own risk of course!!! You could always bash them with a big hammer to break them up a bit.

Grow them, then compost the small trees?

clv101 wrote:Grow them, then compost the small trees?

I wonder if you could grow them on in a polytunnel? Might need some heat over winter.

Growing instructions here

Avocado trees can take up to ten years to bear fruit and indoor-grown plants don't always live that long. However, if you provide it with a moist, fertile soil and plenty of sunlight, and keep it in humid conditions such as a greenhouse or conservatory, your tree will have a fighting chance of fruiting

but

Regardless of region, avocados are not easy fruits to grow. However, the trees make an interesting addition to the conservatory or greenhouse and you’ll never tire of trying to make it flower. If you are lucky enough to get it to bear fruit you’ll be rewarded with delicious, fresh avocados and all your hard effort will be worth it.

I tried growing an avocado stone in Australia using basically the instructions in Ken's link and amazingly it worked.

Six years on and no fruit yet but the tree is about three metres high. You are always told here in Australia that you have to get a tree bought and grafted from a shop to get them to fruit so I will give this one 14 years and then get rid of it. I tried two 'bought' trees and they both died.
It is an amazing thing to see an avocado seed grow as it gradually splits then grows a shoot then a week later a root from the cracks. Even more amazingly the tree lived after planting in compost and the survived me being away from home in a pot with minimal watering.

I am not sure a fresh seed will burn as I dissected one and they were a pulp inside. As for drying and burning why not? Better to crack the shell just in case it explodes though. There is a power station in Queensland that runs on Macadamia nut shells. About 2 MW output I think. Speaking of them I have had good success with Macadamia trees.