If you use flock from flock.com
you will have a tagging bookmark thingie built into the browser. And there are several bookmark addons available for firefox
(for flock as well); however, some of these require an account with one of the many "social bookmarking" sites, for example magnolia, delicious, spurl, gmark, weave, and many more.
I admit I am not very sure whether or not there is an addon that will let you organize bookmarks with tags and doesn't require such a web service.
I used to use delicious, and then switched to magnolia, because their website looks prettier. These days I am using "GMarks" with my firefox 3 beta.
Using an online bookmark storage will let you synchronize your bookmarks too.
But you will have to trust someone with your bookmarks.