The main web site for Firebird shows current development activity, with multiple releases in preparation. The product has a moderately long and slightly checkered history, being derived from Interbase. AFAICT, it is a decent system - it has longevity enough to support that.
I doubt that there are all that many people who specialize in Firebird, certainly compared to, say, MySQL or MS SQL Server, or ... However, I don't think there is anything in Firebird that would be hard for someone who knows one of the mainstream SQL DBMS (commercial or open source) to learn. You'd be looking for a someone with good foundational knowledge of database management and a flexible mindset (knowledge of Firebird a bonus) rather than someone who knows all the details of every version of Firebird ever.