In which version of what did you use that shortcut, and what kind of a sidebar?

Ctrl+H opens and closes the History sidebar and Ctrl+B the Bookmarks sidebar. See View > Sidebar.

@Aleksej: My version of Firefox is 36.0.4 in Linux Ubuntu 14.10. The sidebar that I want to open is the one that has Delicious service inside of it. I suppose other services or kind of things might be in it.

@cor-el: I know that using Ctrl+B gives me the Bookmarks sidebar, or Ctrl-H the History one. Precisely, that is the shortcut that I'm looking for, so I can open|close the Delicious service, or anything else that can be opened|closed with the Sidebar icon.

I'm looking NOT to use the mouse to open|close, but a keyboard shortcut.

I already know that the Customize keyboard plugin does NOT bind a new shortcut, and the Sidebar shortcut is NOT included... or maybe I'm wrong, :D

@cor-el: I should've written '...that's the KIND OF shortcut that I'm looking for,...' I'm sorry for the confussion

@celiapgt: I asked in which version you used F9 for that, because it seemed like they have removed it many years ago.

If an extension opens the sidebar with content then that extension should provide a shortcut to control this, just like Firefox does for the built-in sidebars. I Have the Live Http Headers extension that uses Ctrl+Shift+L to open the sidebar.

@Aleksej: ....I'm sorry, I don't recall what version I used it. If that was removed long time ago, I was stuch with an old version of Firefox.

cor-el gave a hint. It might be the responsability of the developer who creates content for the sidebar to have a shortcut to open|close the content itself... So I will ask the Delicious people to help me in this direction.

Thanks for your guidance! (good questions that pointed me on the "knot" of my issue)