I have to disagree with no sig here; first, many of us work either for handling agents or third-party engineering companies. Does that mean we could never address crewmembers by their first names?
I know many of my "client" crew, based or not, having despatched them for months or in some cases, years. They call me by my first name, even in the presence of PAX; I do the same.
A number of the based crew are friends/acquaintances from a previous career in a now-defunct Midlands-based airline
... am I now to revert to addressing them as Captain/Sir/Madam etc. just because we now have different employers? I think not. If I tried to do so, they would probably ask me "what's wrong with you"?
Same goes for the Cabin Crew; a number of them are ex-colleagues, others are repeat "customers". They address me by my first name after a while. I do the same. I consider THAT respect; if they called me "Sir", I would take it they can't be bothered to remember my name.
As for engineers, it's the title commonly used in Britain; in some continental countries, all aircraft engineers are referred to as "mechanics" since "engineer" has quite a different meaning - even flight engineers are called "flight mechanics". Nothing demeaning there; it's just the accepted term.
Cheers