I call my MIL by her first name. I like her, quite a bit actually, but she really isn’t a “mother figure” to me at all. Much more “friend-like”.
First name.
Jesus, things must really be quite in Bagram if this is what you’re worried about. 
FTR, I call all 3 of my moms (My mom, my bonus mom and my MIL) “mom”.
I call my in-laws by their first names. We have a friendly, but not filial relationship. I can’t imagine calling them Mom and Dad.
I call mine “Ma” or by her firstname, depending on mood and circumstance. He does the same for my mom.
I call my MIL by her first name or by Meemaw, as her grandchildren do. My DIL calls me Babs, and my SIL avoids the issue and doesn’t use a name at all when addressing me. I’m thinking of bringing up the topic and telling him he can call me anything he likes, except “the Wicked Witch of [town I live in.]”
I call her by her first name, Beelzebub.
I call her Eve. That’s her name. She’s not my mom, so she’s not “Mom.” I love her a lot, but she ain’t mom.
Joe
Right now I call her “that woman in the cemetery.” :o
When she was alive, and when I was first married, she wanted me to call her “Mother,” or “Mom,” which I could not do. She did NOT want me to use her first name. I basically avoided the necessity of calling her anything directly, especially as the years went on and what I really wanted to call her was unprintable in polite society. After 9 years when the children came along, I called her “Babushka” a.k.a “Grandmother.”
I don’t call her.
When addressing her, sometimes I’ll call her Mom and sometimes by her first name.
Away from her, I’ll rarely refer to her as Mom to my wife because I do have a still-alive Mom myself and things might get confusing.
To acquaintances, I’ll refer to her as my MIL (since they won’t know her personally, so the first name will be meaning less).
My mother calls my grandmother (her MIL), mom too, but she never really has liked my grandmother much, so I think she just did it because she felt it was expected. Honestly, it never really occured to me to use mom for my MIL, and there’s no way I’m doing it now.
Things are decidedly *not *quiet in Bagram, but hey, now I can post random questions that pop into my head again, and I’m gonna do it, combat zone or not!
I have also been known to call mine “that passive-agressive bitch” from time to time, myself.
I use her first name. She’s not nearly old enough to be my mother. Plus I had a mother and am not eager to move anyone into her slot.
I did my best not to call her anything. I think she would have preferred Mom, but I really couldn’t do that. I already had a mom and I really wasn’t that close to MIL. My husband called my parents by their first names. I think he picked that up from my BIL and from the way they introduced themselves to him.
I used to call her mami, but eventually it became clear that she wanted to browbeat me as if she were my mother (not that my mom does that) without actually, you know, treating me like a person.
So now she’s called by her first name or ‘your mother’ or grandma.
I call both my husband’s mother and step-mother by their first names. I usually try to avoid it though… Was anyone else reminded of this classic exchange from the Dick Van Dyke Show?
I call them by their first names. What bugs me is, my FIL (technically, the husband of my MIL, since he’s not my wife’s biological father) makes a point of calling me “son” every time he sees me. That’s the greeting. “Hey, son.” Stress in original. It’s deeply annoying.
My wife and I laugh at both sets of parents, though, because they always call us “the kids.” We remind them, “Um, we’re over 35 now, and we’ve got a baby of our own, are we really still ‘the kids’?” Apparently we are.
Either her first name or Mom. I get along really well with my mother-not-yet-in-law.