I've been running Firefly on and off for a couple of years. It can load 20,000+_songs to a SoundBridge faster than anything else I've seen.
When I was checking it out, I recall testing version svn-1696 and having some issues. Firefly's site still says that the most stable nightly version is svn-1586. I went with that and it has been mostly trouble-free.
I might suggest trying the 1586 version and see how it works for you.
Also, you can turn on a higher level of logging so that you can see what's going on, by putting, say, debuglevel = 5 into your mt-daapd.conf file.
There is also a database type parameter that you can specify that will cause it to use an sqlite db instead of sqlite3. In fact, the mt-daapd.conf file comments say that sqlite seems to be more stable.
Here's the link to the nightlies including svn-1586.