At the very beginning of the film after the funeral, a woman stands over the open coffin and insults the deceased. She falls into the coffin, the lid closes and she is burned (yes, there were a fire) alive.
It is horror movie, I don’t remember nothing more.
There is a coffin scene at the beginning of Killer party 1986 It isn’t on streaming that I’m aware of
but a guy did a review of it on YT and it shows the scene you are referencing around the 3 min. mark-https://youtu.be/21d6c69Ol5s?feature=shared