How formal is too formal?

During my work experience will it be appropriate to call people Mr X and Mrs Y or should I use their first names?

I’m have some work experience lined up with a smallish local law firm during my half-term holidays and am worried about how to address the staff. Is it appropriate to call people Mr X and Mrs Y or should I use their first names? I don’t want to offend anyone but on the same token I don’t want to sound too formal as people may think I’m boring. Please help.