Together with Xmarks, TagSieve is unbeatable Rated 5 out of 5 stars
This should be standard in FireFox. FF15 broke the link in the bookmarksmenu, but everything else seems to work just fine after you add the blue icon to the toolbar. This is not done automatically. I prefer it this way, because I dislike new addons that clutter by toolbars without asking, but some users might be confused what to do after install.
My ABSOLUTE favorite, is how I can click the blue icon and a list of all my tags shows up. I click a tag and a second menu shows all bookmarks with that tag, plus a new list of all my tags. I choose another tag, and now I only get bookmarks that have BOTH tags.
For example, I have three tags: tutorials, photoshop, software. Depending on the order I click the tags, I can get different selections, such as the following:
Photoshop > software = sites that have software to use with photoshop.
Photoshop > tutorials = sites that have great photoshop tutorials that I bookmarked for future reference
Software > tutorials = software I use to make my own tutorials to post on my blog
Once you realize the power of this system, you will never go back to regular folders again.
Tip: if you don't want to spend days re-tagging hundreds of bookmarks after a systemcrash (html backups do NOT hold tags!), get Xmarks as well. Together, these two are unbeatable!