I am considering the migration for 4 reasons:
1) SQLSERVER installation is a nightmare, expecially for 1-user software (Even if typically I have 3-20 users, sometimes I sell my software to single users: it is incredible to have troubles installing the DB, while installing the applicatino means copying an exe...). (note my max installation is 100 users, but there is no an upper limit). Software installs in 10 seconds, SQLServer in 1 hour. Firebird installation is much easier.
2) SQLSERVER runs on windows server only
3) My customers have all the express edition
4) i am not using any advanced feature, I am now starting using filestream, but the main reason for this is that Express edition has 4/10GB db size limit
So these are all Pros of moving to Firebird.
Which are the cons?
I can also plan to support both platforms, but this will backfire I fear.