The problem is the market was flooded with apps, with almost no quality assurance, which makes it difficult for good apps to shine. Much like various other market places. But with this one it offered a API that was supported by various toolsets and this allowed people to shovel out apps to a almost non-existant market.
Currently all forms of submiting are closed for both addons and apps. With very little to no new apps the market can not expand further. It may as well be dead in the water. With web browser marketshare only being as low as 12% at its current rate it is no wonder why it is.
Keep in mind I am all for the monzilla project and firefox, I still use sea monkey a older various similar to old Firefox.