Hmm, I used to run the spotify client after it launched in firefox v-something single digit. Shame it's such a moving target. Meanwhile, ptitseb's just updated palemoon which is based on the same code I think; maybe that's worth a try?
The only "reliable" spotify on my Pandora (that I know off) is through Android gingerbread but Androids WiFi is messed up, the signal doesn't have enough power and disconnects every now and then.
Luckily the songs and playlists are buffered fast enough to recover. mostly
Obviously I don't often boot to Android to listen to some spotify stream