> I assume on LInux (Debian based Mint version and an Ubuntu flavor) I
> install a variant of the server (in the repository I find the 2.5
> variants, but I'd be also happy installing a downloaded 3.0 instead).
> Then I need a generic DB manager like e.g. Flamerobin to do general
> operations and basic DB editing. Or is it equally possible to do that in
> the terminal?
> Is that correct?
That's mostly correct. 64Bit Firebird might be unable to open old
database - I came to tha Problem with an FB 1.5 database moving to FB 2.5.
In that case you have to install either 32Bit package or an old package.
> How can I determine the version of my existing database file? And how
> can I convert that to a current one in the expected case it isn't
> compatible with my current installation? There is only the .fdb file and
> no backup available.
Once accessible FlameRobin can tell you the version of datafile.
To convert to current database you have to create an backup and restore.