Solved!
I went down the rabbit hole with this and ended up renaming everything to BlackbirdSql, including the DDEX data tools, and re-implementng IServiceProvider in the DBProviderFactory class, then registering it as a second Firebird data provider in the private registry.
That works great so if anyone wants the code you're welcome to it. It targets the most recent .Net Framework version only, net48, but that's a simple change by modifying TargetFrameworks in Directory.Build.Props.
(I've been fairly gung-ho on getting this working so the onus is on you for anyone that decides to use it)
So the long and the short of this is that I previously kept my entity data model
xaml in a separate project using FirebirdSql version 5.12.1, which was
the last version supporting the edmx wizards through IServiceProvider.
When I updated to 9.1.0 I
accidentally updated those too.
I'm not sorry this happened because it
forced me to establish exactly why the edmx tools no longer work and come up with a better solution then constantly having to switch versions.