Bookmark Tags, a Firefox extension, is all the awesomeness of tagged bookmark storage a la del.ico.us, without the hassle of your bookmarks being on someone else’s computer instead of your own, and without sharing your hard-earned bookmarks with the undeserving masses.
I just installed it, so I can’t say whether or not it is great in the long run, but it looks promising.
I’ve pretty much given up on using any other browser than Firefox. There are very good alternatives available on the mac, but I can’t live without my extensions.