This maybe going OT but Firefox actually has daily builds - in case someone wants to be on the bleeding edge
Only their major releases are around 6-12 months apart.
IMO much better than IE that took close to 5 years between version 6 and 7.
The biggest issue with mobile browsers is that of device capabilities. It varies so much across different devices that it is extremely difficult to write cross device browsers with advanced features. So we finally end up having browsers that supports only the common minimum features.