Collation unicode for character set utf8 is not installed
You might get this error when you try to use a database which is created with one Firebird server using one version of ICU library on another Firebird server that uses a different version of ICU library.
If you download the same version (example: 2.1.1) of binary installer package from Firebird's website, then the same version of ICU should be used on all major platforms. This means you can create a database on Windows using Firebird 2.1.1 and copy it to some Linux machine running Firebird 2.1.1 and it will work. However, many Linux distributions provide Firebird binary packages in their repositories, and sometimes the version of ICU they used is different. In such cases you need to backup the database, and restore the backup file on the target server.
You can check the Linux version of ICU library with command 'ldd':
dave@randoms:/opt/firebird/bin$ ldd fb_inet_server | grep icu
libicuuc.so.30 => /opt/firebird/lib/libicuuc.so.30 (0xb787d000)
libicudata.so.30 => /opt/firebird/lib/libicudata.so.30 (0xb7737000)
libicui18n.so.30 => /opt/firebird/lib/libicui18n.so.30 (0xb76f9000)
In this case, it's version 3.0.
On Windows, just look for ICU DLL files in 'bin' directory of your Firebird installation.