{ Ctrl + b }

But how do I fix this?

Once you open it, the Bookmarks Sidebar will stay open until you close it yourself. If you close Firefox with the Sidebar open, it should be open the next time you open Firefox.

That "pin" must have been part of an extension that you had installed, it's never been a standard feature in Firefox.

Not true. When I click on the bookmarks icon (extreme right of the tool bar) I get a drop down menu, but the second I scroll off the menu it disappears. So how do I keep the bookmarks list open. (And the "pin" feature was not an add-on. It was part of the earlier version of Firefox.)

There's two different Bookmarks buttons, the one that you are talking about has a small "drop-mark" arrow which opens a bookmarks drop-down which is like what appears from the Bookmarks menu item on the Menu Bar, when the Menu Bar is hidden that Bookmarks button appears. The other Bookmarks button (without the drop-mark arrow), which is in the Customize Palette by default, is for toggling the Bookmarks Sidebar - which can also be toggled with {Ctrl + B). There's almost two or more ways to do anything in Firefox - menu items or keyboard access-keys or a combination of the two.

If you want that other Bookmarks button see this.
https://support.mozilla.com/en-US/kb/How+to+customize+the+toolbar
http://kb.mozillazine.org/Toolbar_customization

Did you even try using { Ctrl + B } to see what I am talking about?

Go to View /Zoom / Reset. When zoom is too large it will open to the left.