Did it also previously auto-hide if you had opened it using Ctrl+b?

I am not aware of a native feature to auto-hide the sidebar. Could it have been a feature of an extension that was disabled as not compatible with Firefox 23?? You can check for disabled extensions at the bottom of this list (on a gray background):

orange Firefox button (or Tools menu) > Add-ons > Extensions category

Honestly I never used ctrl+b to open it so i do not know.

I checked my extensions and none of them look like they would do it (its all stuff from my anti virus that i had disabled anyway).

I checked my add ons and none of them have anything to do with my bookmark ( i tried turning them on just in case).

Maybe it's the two-very-similar-buttons problem? In the Customize dialog, you can't tell which is the bookmarks sidebar button and which is the bookmarks drop-down menu button. However, when you drag them to a toolbar, the drop-down menu button has the word Bookmarks next to it. So you might need to drag either or both to find the one you want.

Note that the bookmarks drop-down menu button only appears when you are using the Compact menu (orange Firefox button). If the full menu bar is displayed, that button is hidden (since you have an equivalent Bookmarks menu on the menu bar).

I found the drop down menu button. I was only showing one of those buttons in my customize toolbar options. I clicked the restore defaults and both of them showed up.

I must have had an add on i got rid of without realizing it that auto hides when i mouse off of the menu.

Thanks for the help to find the drop down menu, not exactly what i had before but about ten times better than the sidebar.

Ill go ahead and consider this problem solved as you mentioned that autohide is not a native feature.

thanks again.