By their first names and DH does the same with my parents. I think even if we were super close it would be weird calling MIL 'mum' as she's just not my mum, but each to their own.

On a side note, I was getting really irritated when she constantly signed off as 'mum and dad' in my birthday cards, etc, so I made sure I addresed them as both mum & dad (from DH) AND by their first names (from me) in any cards I sent them. She soon got the hint.