Mozilla Firefox users: A question about organizing Bookmarks into folders.

There is most likely a very simple answer to this question.
If there is, I'd be grateful if someone told me.

I have heaps of bookmarked links in my Firefox Bookmarks, but I'm not clear about how to organize them into subject folders.
It was really simple with IE. But I just can't figure it out with Firefox.
There obviously has to be a way to do this, but danged if I know how.

I hereby promise to award one of those "best answer" red ribbon awards to the person who can explain how, in simple English.