They’re gone now, but I always used their first name. Well, once I reached adulthood, anyway. Since I started dating their daughter when I was 15, they were originally addressed as Mr./Mrs.
This was a very awkward thing for me for many years.
The way I was raised, I could definitely not bring myself to call her parents by first name - they said it was fine, I gave it a try, I just can’t do it. It feels tremendously disrespectful.
But of course, they’re too close for me to use formal “Mr.” and “Mrs.” terms of address, and calling them “Mom” and “Dad” felt very forced.
When my wife and I had kids, it got resolved by calling them “Grandma” and “Grandpa”, which are not the terms that our kids call my parents. Whew!
I’ve always called them by their first names. When my FIL was alive, even the grandkids called him Joe and I couldn’t imagine anyone calling him anything else. I still remember when our daughter was little saying, “Hi Doe.” I really miss him.
I called my MIL Maw and his stepdad by his first name. My husband called his mom Mother. He called my parents by their first names.
My FiL is first name. His wife (MiL 2) is first name. My other MiL is a painful relationship. She wanted me to call her Belle-mère (french term) and I just couldn’t. She wasn’t a lovely anything to me. I just didn’t call her anything for years. Eventually the kids came along, and I call her what the kids call her. My husband calls my parents by their first names.
[EdNorton]“Hellooooooo, In-Laws!”[/EdNorton]
Sorry, I’ve been holding that back for two days. Couldn’t hold it any longer.
I called my Mother-in-Law “Ma” which is also how I refer to my own Mother.
In the beginning I used to call my Father-in-Law “Pop” or “Pops” which he liked until I realized what a low down, lying, cheating, self-centered, narcissistic, adulterating rat he is. Now I just call him by his first name Giuseppe which he is neutral on except for the fact I don’t call him pop any more.
If I really want to get under his skin I’ll call him Geppetto, which is a diminutive form of Giuseppe but he freaking hates being called it! Bwahaha! 
I think that’s where it stems from for me. My husband and I started dating when I was 15 and he was 17. I didn’t call his mom anything then!!
My wife had pet names for them, which she used, so I had no trouble using them also, when they were alive. My kids did also. Made life easy.
I do the same unless I can use Gramma First Name when the kids are around!
HA! My brain is still spinning!
When they were alive, I generally managed to avoid having to address them, thereby sidestepping the problem. No, I did NOT avoid having to address them by not being around them, far from it. They were good people and very nice to me. I just managed our conversations such that I didn’t have to address them by name.
I feel very awkward calling them by their first names, even though the spouse and I have been married almost 30 years. For a while I called them “Mr. and Mrs. <Lastname>,” but that felt weird too (also, I didn’t take Spouse’s last name when we got married, so their last name isn’t mine). I don’t feel close enough to call them “Mom and Dad” (especially my FIL, whom I don’t have the highest opinion of) and we don’t have kids so Grandma and Grandpa names are out.
We don’t see them too often, maybe once a year, so mostly I just cheat and don’t call them anything.
This is certainly an interesting thread. I called both of my wife’s parents by their first names, and she called both of my parents by their first names. I don’t recall that we ever thought of anything else. We all were much too close to use Mr. or Mrs. Lastname, and Mom and Dad (or similar) just didn’t feel right to either my wife or I. I never thought of first names being problematic, but my wife and I always spent a lot of time with both sets of parents.
Perhaps a tangent, but once I became about 18 or 20, I don’t recall calling any friend’s parents Mr. or Mrs. Lastname. Very few of my kids friends have ever called me Mr. Lastname or my wife Mrs. Lastname once they were in middle or high school. When our kids were in elementary school, we did have them call our friends Mr. Firstname or Miss Firstname, but that was phased out by high school.
As I posted in that thread, I used the same names/terms their two kids did – Muvver and Popsi.
First names.
It helps that since I’m nearly 11 years older than my wife, my MIL was only 9 years older than me, and my FIL is only about 12.5 years older than me. So they’re not my parents’ generation, more in between. (In fact, my wife’s grandparents were about the same age as my parents.)
First names. But I do call MIL Maaaa when I want to annoy her.
They’re both deceased, but when they were alive, I don’t recall that I ever addressed my FIL. He worked evenings, which was mostly when I was there, and fairly soon they divorced. My wife wanted nothing to do with him, so we seldom saw him.
My MIL had a special nickname that we called her. It was only used by family so I used that. When the first grandchild came along, I called her Grandma “nickname”.
MIL was dead before I met ex-wife. FIL was, and is, “Gene”, if I ever speak with him again.
It’s all good.
When my wife and were about to marry 20 years ago, my in-laws explicitly stated they were to be called Mom & Dad. I hated it then and still do. I already have a Mom & Dad, and it seemed archaic and unnecessarily formal to call my in-laws that. Their justification was that, since my wife’s other siblings were already married, that’s what those spouses did.
Although my parents are the exact same ages as my in-laws, they’re more progressive and told my wife to call them by their first names.