My monitor is plenty wide, so I have FF set to keep an open section (sidebar) on the left side with the list of bookmarks. To bookmark a webpage, it's simple:
I move cursor arrow to the tab of the page I want to bookmark
I hold the left mouse button down
I move the mouse, dragging the tab to whatever folder I want it in, and
release the button. There it is now, in the folder.
Now, if you don't know how to make the bookmarks sidebar appear, here you go. Click on the icon to the right of the star... the icon for "show your bookmarks". When the drop-down menu appears, the first thing at the top should be "view bookmarks sidebar". Click it. Sidebar appears!
(edit: 1492, you beat me to it while I was typing!)