I'm not really sure how much the increased marketshare of Firefox is affecting number of defects reported. They get louder press now due to visibility I suppose, but we've been finding stuff since the browser was first released. The code is still fairly new, but I'd say it's become pretty solid overall, at least by my usage. I've been using FF since it was a .2 release. (off and on - some of the early nightlies were horrid.) Relative to other offerings out there, it's not doing too badly as far as features, stability and defects are concerned IMO.