Sorry for my english :)
Bookmarking is about retrieving informations. You browse, then find an interesting article about… whatever and you want to save his location for a later revision.
The current paradigm for bookmarks is based on a common interface, the folder/entry structure that you use for browsing trough the files on normal filesystems.
That’s a good method, since you can organize your bookmarks into categories, sub-categories, and so on.
Categorization is good, but often you need more flexibility. Maybe you want one of your bookmarks to be accessible both from the “Graphics” and the “Webdesign” folder, then you should copy the bookmark in both of them, and we all know that you should not repeat yourself :)
As a solution to this issue, Mozilla Firefox 3 introduces bookmark tagging: you can assign multiple small text labels to each bookmark and use them to improve the categorization.
I see two big issues in Mozilla Firefox current bookmarking model: managing bookmarks and accessing bookmarks.
Managing bookmarks includes both folder structure (that I think is very useful for giving a general structuration to your bookmarks) and tags. The current folder structure works pretty well, it’s a solid interface design that could be used as a fallback for every user.
About tagging I find that current interface could be improved a lot:
Knowledge management is pretty useless if you don’t have a good interface to retrieve stored information.
Excellent Dria toughts on tagging
Autotag/suggest bookmarks from popular delicious tags
Samuel Slider on bookmarking and tagging
Robert Accettura's Intelligent Bookmarking Draft