If users never use the options available, it would be nice to be able to hide them in some way. Maybe through about:config, or in the "Organize Bookmarks" section, or a new "Bookmarks" section in the Options menu (like the Tabs section).
I feel this feature request is in line with the ability to hide tabs when only one tab is in use (which users can choose through Tools → Options → Tabs → Always show the tab bar).
The ability to hide Bookmarks → Unsorted Bookmarks if there are no bookmarks currently in that section would be AMAZING and highly appreciated. I feel the technical details of the implementation would not be too difficult.
The ability to hide Bookmarks → Subscribe to This Page would be another nice thing to have. Many of us don't participate in RSS feeds and the menu option is superfluous for us.
The ability to hide Bookmarks → Bookmark All Tabs would be another nice thing to have. Many of us don't ever use this.
The ability to hide Bookmarks → Bookmarks Toolbar if there are no bookmarks currently in that section would be very nice as well. Again, I feel the technical details of the implementation would not be too difficult. Users who don't use the bookmarks toolbar should be able to hide this.
It would be nice as well to remove the horizontal rules between the sections if the entire section does not exist, or maybe in some other configurable method. Getting rid of the menu items doesn't help much if the horizontal rule doesn't go away as well.
Here is an example of what my Bookmarks menu looks like:

And here is what I'd like my Bookmarks menu to look like:

For those of you keeping track, that's 4 menu choices I'd like to hide, and 2 horizontal rules I'd like to no longer see.
Here they are, side-by-side:



