Gnucash is an actual double entry bookkeeping system. Firefly is not.

I want to like Firefly, but you cannot properly record all liabilities. Also, you can't mix income and expenses in the same account. For example, if I get a refund, it has to go to an income account. Not net off against an expense, which is the usual way bookkeeping.

Still a lot of depth to Firefly and has its own learning curve.

Its also written in PHP using Laravel which can be appealing.