Everything is in the subject line ...
There are a large number of models of the Galaxy Ace http://pdadb.net/index.php?m=search&exp=galaxy%20ace
Only some of them meet the minimum specs we want to target for Firefox 16. We are currently looking at supporting ARMv6 phones that have 800mhz 512mb ram. Thus support depends on the exact model of the Galaxy Ace you have.
This page explicitly lists the Galaxy Ace GT-S5830 as supported:
I have the GT-S5830i, and it appears there's also a GT-S5830T, but neither of them appear to have 512MB of RAM. Perhaps that page is wrong?