Response I received from Amazon regarding tabbed browsing:
"At this time, the feature of advanced browsing is not available. We're regularly working on improvements to your Kindle experience.
Customer feedback like yours helps us continue to improve the service we provide, and we're glad you took time to write to us. I've let the Kindle team know you're interested in this feature. "
Pretty non-committal, but it can't hurt for anyone interested in this feature to continue sending inquiries to
kindle-feedback@amazon.com