This required a script to replace the libicu supplied by bullseye with the preferred one for the relevant firebird versions. If for some reason you do get a Collation unicode for character set utf8 is not installed error you can use gfix -icu <database> to correct the issue as of firebird 3.0.
Both 2.5 branches are now included as well. Unfortunately the gfix -icu <database> option is not available for 2.5 so instead I have opted to add tags for v2.5.9-sc-jessie and v2.5.9-ss-jessie If you find your setup works with the jessie tags but not the newer v2.5.9-sc or v2.5.9-ss tags please switch back to the jessie tags and open an issue to let me know. This will probably be the last major update for v2.5 as version 2.5 has been discontinued for 2 years now.