I don't think I would ever have thought this expression meant "bonfire" except that the Tips and Notes put the connection into my head by telling me that it doesn't... and now I keep getting it wrong.
This expression is for the 'fireworks, not the bonfire. It is literally 'wild fire' which I'm sure you'll agree is a great description of fireworks.
Bonfire is 'coelcerth'
Nov 5th in Welsh is 'Noson Tân Gwyllt' = Firework night
Fireworks seem very controlled and not at all wild to me. Maybe we have different ones here than you have there.
This whole discussion is very confusing for one who doesn't know the culture. Is there also a Bonfire Night that also occurs on Nov 5?
Bonfire night and fireworks night are different names in the UK for the same thing.
Lighting bonfires and setting off fireworks are part of the celebration.
As rmcode says 'gwyllt' is wild. 'Hwyaden wyllt' literally 'wild duck' is used for mallard.
We say Bonfire night and sometimes Fireworks night.
I wonder what the word for 'a wildfire' is yn Cymraeg?
geiriaduracademi.org will give you the answer...!
I looked up wildfire and its tân gwyllt. Bonfire is coelcerth.