Firebird 1.0 was based on Interbase 6.5, so the two are pretty much
identical featurewise, although FB 1.0 is a lot more bug free than IB
6.5.

Firebird was then ported to C++ and the current version 1.5 is a
result of that. Featurewise, all the new ones are in the release
notes, and there are some good features that were added, as well as
some more bug fixes and optimisations.

Firebird 2 is now in alpha with even more optimisations and features
(I will not bore you with them all, but they can be read in the
release notes for FB2).

I do not know what features Interbase introduced post 6.5, but if you
don't use them then the conversion will be pretty simple. The main
differences is that Firebird will give you an error in some cases
where the query is ambiguous, but if your queries are not ambiguous
then this will not be an issue.

As for installation, you do not have to install anything first.

The main website for Firebird is:
firebird.sourceforge.net

You can download a setup.exe for windows distributions, or different
rpms etc for linux-ish distributions.

The sequence in windows is:

1. Download the installer,
2. Double click it.
3. Next, Next, Next, Next, Finish

:)

It can co-exist with Interbase, BUT they will both want to use port
3050. You can tell Firebird to use a different port with a simple
setting in Firebird.conf if this is the case.

You might as well give it a try, if it works for you then you have
saved yourself a bit of money.

Adam