I call them by their first names, which is what they prefer. It’s been five years, so I’m pretty comfortable with it now, but in the first year I kept having to stop myself from calling them Mr. and Mrs [Lastname].
I’m just not a very formal person… I have a really hard time calling anyone besides teachers “Mr./Mrs. Lastname”
I think I’d have a hard time having much respect for someone who insisted on being called that way, too, in anything other than a professional relationship. It would just seem pompous and self-important to me.
First names. I started with Mr./Mrs. Lastname but they told me they hated it.
Can’t imagine calling my Japanese inlaws by their first names. FIL has NEVER EVER in 55 years of marriage used his wife’s name! Nor her, his. I call them “mother” and “father” in Japanese (okaasan, otousan) if I have to but I skirt round it by addressing them via the kids as “Granny” and “Grampa” (Baachan, Jiichan) when I can get away with it. They use my given name and no “san” which is nice. They use “san” for my brother in law’s name though, even though they live together.
First names. When we were first dating, I used "Mr and Mrs…,"but they aren’t formal people and that didn’t last.
They tend to get called “grandma and grandpa” a lot these days too.
Him: His first name.
Her: Betty. (old joke, based on “Helloooooo, Betty.”)
I voted in the other poll and I shouldn’t have either. 13 years here.
I call them Mr. & Mrs., which they certainly don’t insist on, but Mother and Father isn’t right, and the other daughter in law calls them Mom and Dad, but she’s married, and we don’t do first names for the elder generation in Asian families.
But mostly I do my best not to ever call them, though I don’t mind writing Mr. & Mrs.
“Suegra.” It started as a joke, but now it’s a good balance between friendliness and respect.
Nancy and Mama N.
I get that with parents of kids I grew up with. I never know how to address them, since I grew up thinking of them as “REAL LIVE ADULTS!” (you know how kids think that teachers and their friend’s parents don’t have first names?)
I grew up calling all adults except teachers and doctors (and family members like ‘mom’ and ‘dad’) by their first names.
I always addressed my wife’s parents by their first names. I liked them and was perfectly comfortable around them. They’re both long dead, unfortunately.
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We call each other “DOL” and “MOL”. That’s Daughter-Out-Law and Mother-Out-Law… well, if being married would make us in-laws, then…
that’s awesome!
Me too. Even when I worked for multi-millionaire CEOs it never occurred to me to call them anything but their first name.
I call my in-laws by their first name.
I call my fiancé’s mother usually by her first name, Brenda, though I am very uncomfortable doing so. She’s a perfectly lovely woman, but I slip back into Mrs. Lastname. It’s very formal, but it’s how I was raised. His father died before he was born 
My fiancé says he would call my mother and father Mom and Dad if he ever had to address them by something. Luckily he’s never had to…they might be weirded out by that at first 
I still call my ex-Mother-in-Law “Mum” as I did when I was married to her son. We had a very close relationship that has not dimmed since the divorce (as I’ve said before, she “adopted” me after the divorce). I call my ex-Stepfather-in-Law by his first name, which he prefers.
Until they told me otherwise, Muvver and Popsi (My late BiL grew up with a speech defect and “his” names just sort of stuck) were Mrs and Mr Theirname. They used their own names so rarely (first names) that calling them so meant something very serious. When I had to tell Muvver Popsi had some serious health issues we all needed to work together on the conversation started; “Lou - listen for a minute”. This was a keyword (sort of) to say the following had to be told straight through once and then dissected from there.
My Dad was Mr Ourname until he told my wife otherwise; at that point he became Dad.
My mother was always Mother to me, my wife and my Dad.
The way we pronounce it, the word “You” is clearly implied.
I like your choice but ours is faster and easier to say in mixed company. 
I called my late SO’s father Pops, the same as he did. His step-mom, mom and step-dad were Miss / Mr. firstname. His grandmothers were Memaw and Granny.
The rare times that he was around my parents, they were Mr. / Miss Firstname.
I call my MIL “Mum.”
I call my FIL “Pop”, “Poppy”, “Pap”, “Pappy”, “Dad”, “Daddy”, “Big Daddy”, “Father”, “Popster”, “Pastor” (He’s an ordained minister), “Padre”, or “Paddy”, depending on the situation.
First name, though I generally try to find just about any kind of conversational workaround short of setting myself on fire to avoid using a name at all.