You should do some tests and ask some more specific questions. You
cannot reasonably make decisions like this without experimentation. This
community is full of people that have found Firebird to be a VERY fast
solution - but usually for applications that require ACID performance.
Bulk inserting is not its forte because you pay a performance price for
the other excellent attributes Firebird has. Transactions, which you say
you do not need, are an important part of the rock-solid operations you
typically get using Firebird.

Sounds to me that Firebird is probably not what you are looking for but
you will not know for sure until you try it out.