Firefox:3.0 Bookmarks
Smart Bookmarks - I'm sure everyone has read what I've read, but it doesn't hurt to write a reminder. Here's a link to UIWeb's "How to build a better web browser" essay: http://www.scottberkun.com/essays/essay37.htm
See also: Firefox:History
Relevant research (this is 5 years old but never used!):
Integrating Back, Bookmarks and History - Alternative History Mechanisms for Web Browsers:
http://www.cs.umd.edu/hcil/about/events/history-workshop/slides/Kaasten/ (Online presentation) http://www.cpsc.ucalgary.ca/grouplab/papers/2001/01-IntegratedHistory.CHI/00-IntegratedHistory-CHI.pdf (PDF paper)
Initial first steps towards a Bookmarks Data API. The API needs to be updated, based on Bookmarks Design Discussion.
If you'd like to describe something you would like to do with Bookmarks, add it to the Bookmarks Use Cases page.