{Ctrl + B} or View > Sidebar > Bookmarks

Or, try this to get the bookmarks dropdown menu on the left like it was in FF 3.5 & 3.6:

 View>Toolbars>Customize 
 (or instead, R click anywhere in the lower part of the toolbar>Customize)

     The Customize Toolbar window opens.

 Drag  the Bookmarks button on the left side (hover over it and left click - a 'hand' will grab it) off the toolbar and into the Customize Toolbar window and drop it anywhere inside there...    leave the Customize Toolbar window open.

 In the same way, grab the Bookmarks button on the right side then Drag and drop it onto the left side of the toolbar, to the same place you just removed the other bookmarks button from.

 You now have the same functionality that 3.x had - dropdown access, with a single click, of all your bookmarks, and it closes when you click a bookmark to open a webpage.  Seems identical to the old versions.  You still have access to "organize bookmarks" by clicking "Show All Bookmarks," also like before...

 If you want, you can also drag the "Home" button to a spot next to it.  If you do, the right end of the search bar or navigation bar is then nearly touching the right edge - if it's too close for your esthetics you can place a "space" by dragging one from the Customize Toolbar window to that edge.

 I tried to get used to the Home and (especially) Bookmarks buttons on the right, but to me it's counterintuitive.  Moving them back to the left works better for me.  Fortunately Firefox has a lot of toolbar customizability.

                       Ben Roesch

Here's an idea. The simple 'X' is used to close unwanted features that people accidentally pushing things open.

Why did I have to load up firefox and look up how to close the left side bookmarks bar because my mom opened it and wanted it turned off?

How difficult is it to put a simple CLOSE option in on a right click, or an orange X (like everyone is used to), or something?

I love firefox, but I like to see the web pages I open. If I want bookmarks I'll click on them. The last thing I need if I recommend a browser to someone is something that is more difficult than Internet Explorer. I don't want to do more work Mozilla, please at least make user f*ck ups as easy to fix as they are in IE.

There is supposed to be an X is the upper-right corner of the Bookmarks Sidebar, for closing it. If that X isn't there, maybe you have an add-on installed that hid that X.