The one, and perhaps only true weakness of Firefox is in its bookmark system. It's not intuitive, requires a fair amount of clicking through options and menus and doesn't follow standard industry practices.
Can we revise the bookmark system in Firefox so that you can eliminate a duplicate menu with a simple click on an option? Or edit the bookmarks by dragging and dropping them, without opening them? Or editing them with a right click? Or edit each bookmark title with a drop down when hovering or left-clicking on it?
Keep it simple...
