“Also, some poskim say the same for a deceased father who was a bechor.” What are you talking about or was this a typo?
No, not a typo. However, this very old minhag (approximately 700 years) seems to have expanded in some circles, including the aforementioned addition of the deceased father. If you are looking for the original minhag, then
1. Women first born also need to fast
2. Non-bechorim who are hygienically scrupulous (איסטניסים) also fast.
How many abide by the original minhag?